This might be caused by the installation of Kee onto a Firefox profile that used to have KeeFox installed but did not receive updates for many years.
Please check which version of KeePassRPC you are running:
- In KeePass, go to the Tools menu and click “Plugins…”
- In the dialog that loads, find the entry for KeePassRPC and verify that it is at least 1.7.3.0
If it’s lower (older) than that, please follow the instructions in Upgrading KeePassRPC to get the latest version.
If that doesn’t work, the Kee log can be enabled from Kee Options. It defaults to “Warnings” which might already be enough depending on what is going wrong but you may need to enable Information or Debug mode.
You can then view the log by opening the browser console (Ctrl + Shift + J). It gets mixed in with all sorts of other log entries unfortunately but hopefully Firefox will improve this one day.
Sorry for the slow response - there wasn’t really much to go on from your message and it’s only in the past few days I’ve seen any other example of this “unexpected error” occurring.
It’s also probably worth considering refreshing your Firefox profile if the above does not help - it could have become corrupt over the years and be causing strange bugs that affect only some add-ons.