If that still happens when you rename the .htaccess to something else (to rule out it's still being used), than the issue is server side (assuming your code doesn't produce a 500 error. The framework doesn't).
I have no plesk experience, and don't know the setup of your server, so I wouldn't know what the issue could be.
I found it out. Just had to move to the correct /public folder in Plesk BY Plesk. :-) After upgrading to the new version it was hard to find where to make it.