Okay, this actually just boggles my mind.
I re-installed phpBB3 with fresh files from here, fresh database, downloaded de metrolike style again (so it's a clean file, no edits) and uploaded it with ftp. I can activate it, but when I use it I get an error page.
This is the error:
HTTP ERROR 500
This is the link to my forum. If you to 'your first forum' you should get the error.
I have no clue where to look.