This has just happened to me - for my forum page only. It happened directly after adding the 'Who has visited in last 24hrs' addon. Even after uninstall still get the same issue, with no server errors. I have added the line in this thread to try and see if I can get a view of the error but no luck.
Very strange - a blank index.html page was installed on my root somehow???
Deleted it and it is fine now. This was a new issue today and not part of an install of xenforo but must have happened when istalling an addon... It happened directly after installing 'Who has visited in the last 24hrs'