Concerns the Fedora Flatpak (from registry.fedoraproject.org). The Flathub Flatpak works.

The problem might be due to the Fedora Flatpak being build from the RPM. In the Flathub Flatpak of KeePassXC a wrapper script decides whether to start the KeePassXC proxy or the application itself. It does this based on command line parameters coming in.

In the Fedora Flatpak the wrapper script does not exist within /app/bin and thus it is always tried to start a new KeePassXC instance instead of the proxy when the browser initiates a connection. This then results in the message that a instance of KeePassXC is already running.

Reproducible: Always

Steps to Reproduce:
1. Install KeePassXC Flatpak from registry.fedoraproject.org.
2. Install the KeePassXC Browser extension in Firefox.
3. Open a database file in KeePassXC and activate Browser integration in the settings (select Firefox as browser).
4. Click on the Addon symbol in the browser and click "Reload".
Actual Results:  
The browser plugin reports "Key exchange was not successful." and the log shows "Another instance of KeePassXC is already running.".

Expected Results:  
Browser to KeePassXC Flatpak communication should work.

See also https://github.com/keepassxreboot/keepassxc/discussions/11598 for more detailed information.