This issue shouldn’t happen unless the wiki has been upgraded. Do you have knowledge of any recent upgrade? If yes, knowing the versions would help 
In the meantime, if you urgently need to restart your server and have access to the filesystem, you can move/remove the mention sub-directory of your permanent directory (see the Configuration guide).
Warning: This directory contains a file that stores a queue of unprocessed mention related events. By removing this directory some user mentions could never reach the notifications inbox of the mentioned user (i.e., if the queue was not empty when the server was stopped). Please proceed with caution.