When I did my first evaluations and test of wpforu I tested image uploads and they worked great. I am unsure when they stopped working.
Ok, turn also on Wordpress debug ON and debug to a log to ON (true)
// Enable WP_DEBUG mode define( 'WP_DEBUG', true ); // Enable Debug logging to the /wp-content/debug.log file define( 'WP_DEBUG_LOG', true );
And when you do what it doesn't work, press (before) F12 and see your browser's console.
If all those are clear, then it could be a permission issue, ask your hosting company to see the real error and fix it.
I have turned on WP debug and only got a few deprecated define errors in my theme. Nothing that would indicate an error with attachment uploads. It seems strange to get a 500 error and not see it in the WP debug error logs. I suppose it must be a hosting error. I will check there next.