when you have updated the plugin you cannot simply overwrite the plugin files with an old version. it will not work. you must restore from a backup and re-do the upgrade (as it clearly explained in the upgrade notice / howto)
first of all though you should find out what the 500 error was and make sure this does not happen again
and you MUST update all extensions to v3 compatible versions (as is also stated in the update notice/guide)
