Have you cleared the caches? See https://docs.moodle.org/33/en/Developer_tools#Purge_all_caches
Have you got full debugging switched on? It would be helpful to see more information about the error. See https://docs.moodle.org/33/en/Developer_tools#Debugging
That aside though, v3.3 is now very out of date and no longer supported so you should consider upgrading.