We'd love to have your feedback on the action creation and submission process. We try to improve the process on our side with every release, but there's little we can do on the Facebook end of things.
It seems the action you took on your end changed the permissions or the extension you work on requires higher ( then 755 ) privileges.
JFBConnect doesn't change file permissions at all. We also don't require permissions other than the ability for Joomla/Apache to read the PHP files to be able to execute them. There are no writes to the file system when a user takes an action, it all takes place in your database. So I'm not sure what permission issues you may be having.
The 500 error message above is pretty generic and just has recommendations. The problem could be a PHP issue, an outgoing connection to Facebook (to post the action) issue, server connectivity issue or something else. If you can get a hold of the PHP error log, that'd be the best way to narrow down if there's something in JFBConnect that isn't being handled properly and causing a PHP error. The next step would be to check the Apache logs to see why the 500 error is thrown.
Finally, the 500 error message above doesn't look like a standard Joomla error... it may be from your server itself, in which case it's almost definitely a server connection/resource issue.
I hope that helps,
Alex