Browser and cache issues

If this is limited to one user, the most common culprit is a corrupted browser cache or a conflicting extension. Focus on these steps first:

• Firefox Private Window: Have the user log in using a Private Window (Ctrl + Shift + P). If it works there, a browser extension or cached site data is the cause.
• Clear Nextcloud Site Data: Open Settings → Privacy & Security → Cookies and Site Data → Manage Data. Search for your Nextcloud domain and remove it.
• Tracking Protection: Click the shield icon in the Firefox address bar and toggle off Enhanced Tracking Protection specifically for your Nextcloud instance. Firefox can occasionally be more aggressive than other browsers with blocking the cross-origin requests required for Collabora.

File locks and session conflicts

Nextcloud uses a database-level locking system to prevent simultaneous edits. Sometimes these locks do not clear properly:

• Identify the file: If this only happens with one specific file, try to rename or move the file within Nextcloud. If it still fails, the file might have a ghost lock.
• Check for session locks: If the user has the document "open" in a different tab, or on another device that they didn't close properly, Collabora may prevent a new session. Ensure they are signed out on all devices.
• Clear the oc_file_locks table: If you have administrative access to the Nextcloud database, you can manually clear the file locks table, though this should be done with caution (ensure no one else is currently editing).

Nextcloud user profile

Occasionally, a user's specific settings or session token can cause issues:

Logout/Login: A simple logout and login will refresh the session token.

Change User Theme/Accessibility Settings: There was a known bug where specific high-contrast or accessibility themes in Nextcloud caused JavaScript errors in the office integration. Revert the user to the "Default" appearance.

Test with a clean local profile: Ask the user to try another browser on the same machine (e.g., Safari or Edge).

• If it works in another browser, the issue is strictly limited to their Firefox profile.
• If it fails in the other browser too, the issue is on the server side tied to their Nextcloud account (permissions or database entry).

Further Diagnostics

Ask the user to attempt to open the file whilst looking at both the Nextcloud log (JSON) and the collabora container log. This will in many cases reveal the cause of the issue, or at least point you in the right direction.