Hello mlorch,
Sorry your having this issue however I don’t have enough information to tell much. First, it sounds like a javascript error since the body is hidden at start and shown after the javascript is run. With that said, which browser are you using?
If it is in fact a javascript issue than you should have errors in the browser. If you are using Chrome for example, you can right click and push Inspect to view the console (anything in red is an error).
It also might be a missing PHP extension. If that’s the case, you will have errors in your apache log (in /var/logs/apache2/error.log).
If you find any errors in either location please share them with me so I can track down this issue.
Thanks!