Make sure you are running the latest version of KeePass and KeePassRPC and that should fix this problem for you.
https://keepass.info/download.html
Seems that the latest versions of Firefox make it a bit harder to find the version number of Kee.
Go to about:addons and click on the “Kee” title/name; then scroll right to the bottom and look for the Version number there.
It’s probably automatically updated (to version 2.4.x) anyway.