It appears that apache is sending 404 status sometimes when someone performs a "GET /" request. Sometimes it works though.
For the cases it doesn't, visitors are presented with a blank page. Any other part of the site works. You can check www.f1technical.net as sometimes failing, and www.f1technical.net/forum as always working.
I have tried replacing the homepage with a simple "hello" in the file, but that doesn't matter. I tried a lot but don't really know where to look further.
Any guesses what might be causing this?
EDIT: It turns out that this was caused by an incompatibility in php code.
Problem is really that PHP errors are not logged correctly, so I have been looking at the wrong place all the time.
Thank you for your contributions!