Troubleshooting the Outlook Add-in#
This page covers the classic Outlook Add-in, installed with an MSI. Not sure which one you have? See Which add-in do I have?.
Start with these#
- Close Outlook completely and start it again.
- Check that the add-in is switched on, under File > Options > Add-ins. Make sure it is not on the disabled or slow add-ins list.
- Reinstall with the latest installer, as administrator. This writes the registry values again from scratch.
- Sign in to the FileCap portal in a browser with the same account, to confirm the account itself works.
Report a problem#
From version 2026.10.0 the add-in can prepare the support mail for you.
- Open a new message and click the gear (Settings) icon in the FileCap panel.
- Click the Version number line three times in quick succession. A log level dropdown and a Report a problem button appear.
- Set the dropdown to DEBUG, reproduce the problem, then come back here.
- Click Report a problem. Outlook opens a draft to support@filecap.com with the add-in, Windows, Outlook and .NET Framework versions, Outlook bitness, the add-in and Outlook languages, the portal and the log level already filled in.
- Fill in What went wrong and Steps to reproduce, attach the exported log (see Exporting the log) and send it.
The dropdown has three levels, ERROR, INFO and DEBUG, and writes
the DebugMode registry value for you. While the level is not ERROR, a red
“Loglevel is: …” line shows at the top of the panel. Click click here to
reset on that line to go back to ERROR when you are done.
On older versions, or when the add-in does not start at all, use the manual template at the bottom of this page.
The log#
The add-in writes to the Windows event log:
Event Viewer > Windows Logs > Application, source FileCap.
The default level is ERROR, so errors are recorded without changing anything
first. For more detail, raise the level with the dropdown above or with
DebugMode in the registry (see Registry settings),
restart Outlook and reproduce the problem.
Exporting the log#
Right-click the Application log, choose Save All Events As, and save it as a
.evtx file.
Messages and what to do#
| Message the user sees | Cause | Fix |
|---|---|---|
| “The mail body format is not in the right format. Please change the format to HTML” | The message is in Plain Text or Rich Text. FileCap only sends HTML messages. | In the message, choose Format Text > HTML. To stop it recurring, set File > Options > Mail > Compose messages in this format to HTML. |
| “You have attached a 0Kb file to this message. This cannot be sent with FileCap.” (or “several 0Kb files”) | An attachment is empty. This is often a cloud file that was not downloaded yet, or a file still being written. | Open the file to check it has content, save it locally and attach it again. |
| “… This message cannot be sent due to a prohibited file extension.” | The portal administrator blocks this file type. The same message shows when the add-in could not reach the server to check the file. | Check the blocked extensions in the portal settings. If the type is allowed, check the connection to the server and the log. Otherwise zip the file or send it another way. |
| “A virus was found in one of the attachments. Sending this message is not allowed.” | The server’s virus scan flagged an attachment. | Do not resend the file. Scan it locally and involve your security team. If you are sure it is a false positive, report it to us with the file name. |
| “This device is not associated to a FileCap account. This message cannot be sent.” | The registration of this Outlook with the user’s FileCap account is missing or was removed, for example after the device was deleted in the portal. | Register the device again from the FileCap panel (Register device, then paste the code from the email). If that fails, delete the OutlookUserAPIKey value for this sender under HKEY_CURRENT_USER\Software\FileCap\Users, restart Outlook and register again. |
| “FileCap is currently being activated. The message will be saved in “drafts”. …” | The portal requires device registration and this sender has not completed it. The message is saved in Drafts. | Complete the registration in the FileCap panel, then send the message again from Drafts. |
| “The following attachment(s) cannot be added due to file size restrictions: … Please re-attach the files before sending this message from “Drafts”.” | A message with large FileCap attachments was saved as a draft. Those attachments are not kept in the draft. | Open the draft, attach the listed files again and send. |
| “A secure SSL/TLS connection cannot be established.” | The add-in only connects with TLS 1.2. The certificate chain is not trusted on this PC, a proxy or SSL inspection replaces the certificate, or the server does not offer TLS 1.2. | Open https://<your Server value> in a browser on the same PC and check the certificate. Exclude the FileCap host from SSL inspection, or make sure its root certificate is trusted. |
| “An unsupported FileCap version was detected.” with “The FileCap plugin needs to be updated.” | The portal no longer accepts this add-in version. FileCap does not load. | Install the latest add-in. “Update available for the FileCap plugin.” is the softer variant: the add-in still works, but update soon. |
| “The FileCap licence is invalid or has expired. …” (in the FileCap panel) | The licence of the FileCap environment is invalid or expired. | Contact the administrator of the FileCap environment, or your reseller. Nothing on the PC fixes this. |
| “Warning: the FileCap server cannot be reached.” | The add-in could not reach the server at start-up or when sending: no network, proxy or firewall, wrong Server value, or the portal is down. | Open https://<your Server value> in a browser on the same PC. Check proxy and firewall rules and the Server value. Restart Outlook once the server is reachable. |
| “FileCap is not available.” when sending | FileCap did not load at start-up, usually because the server was unreachable or the registry values are wrong, and AllowFailedConnection is not true. The message is not sent. | Fix the start-up problem (see the log and Registry settings) and restart Outlook. |
| “This message cannot be sent because there is no connection with the FileCap Server. …” | The server cannot be reached and AllowFailedConnection is true, so the user may send without FileCap. | Choose Continue sending to send the message as normal email, without FileCap protection, or Cancel to go back. See Specific settings for AllowFailedConnection. |
| “The DNS functionality has been disabled for this plugin version. …” | The APIKey registry value is missing or empty. | Fill in APIKey (see Registry settings) and restart Outlook. |
| “It is prohibited to send messages with FileCap from this email address.” | The sender’s address is not in a domain allowed for this portal. | Send from an allowed address, or have the portal administrator add the domain. |
| “The files cannot be sent because they are in use by another process.” | Another program has the file open. | Close the file in the other program and send again. |
Known conflict: server-side signatures#
Exclaimer and other server-side signature solutions are a known source of conflicts with the classic add-in. If one is in use, mention it when you report the problem, as it changes where we look.
Registry settings#
A missing or wrong Server or APIKey makes the add-in fail as it starts. An
error such as “Object reference not set to an instance of an object” right after
installation points here first.
The add-in reads these keys, in this order. The installer writes to
HKEY_LOCAL_MACHINE by default.
HKEY_LOCAL_MACHINE\SOFTWARE\FileCap
HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node\FileCap (32-bit Outlook on 64-bit Windows)
HKEY_CURRENT_USER\Software\FileCapA value in HKEY_CURRENT_USER overrides the same value in
HKEY_LOCAL_MACHINE, so check both. The installer writes Server, APIKey,
Language and DebugMode to HKEY_CURRENT_USER of the account that ran it as
well, and the language and log level dropdowns write Language and DebugMode
there too. After changing a value in HKEY_LOCAL_MACHINE, for example with a
Group Policy, update or delete the same value under
HKEY_CURRENT_USER\Software\FileCap, or the old value stays in effect. All
values are listed under Specific settings.
| Name | Type | Value |
|---|---|---|
Server | REG_SZ | The hostname, for example filecap.company.com |
APIKey | REG_SZ | The key issued for that portal |
DebugMode | REG_SZ | 0 Error, 1 Informational, 2 Debug. A missing or unrecognised value means Error |
Check that:
Serveris the hostname, for examplefilecap.company.com. From version 2026.10.0 the add-in removeshttps://, a trailing slash and surrounding spaces itself. Older versions need the hostname only.APIKeyhas no stray spaces from copying and pasting.- The API key belongs to this portal. On an MSP setup, each sub-portal has its own key.
What to send us#
If the steps above do not resolve it, send the details below to support@filecap.com. With Report a problem most of this is filled in already. Click the block to copy it.
FileCap Outlook Add-in (classic)
Add-in version:
Outlook version and build: (File > Office Account > About Outlook)
Outlook bitness: 32-bit / 64-bit
Windows version:
Server value from the registry:
APIKey filled in: yes / no
Server-side signature tool: Exclaimer / other / none
Exact error message:
What goes wrong, and when: (opening the add-in / signing in /
adding an attachment / sending)
Every time, or now and then:
One user, or several:
Attached: the exported .evtx, recorded with the log level set to DEBUGPlease do not send the API key itself. Confirming that it is filled in is enough.