what can cause login.php (not max query)

Hello,

I am very frustrated. The page is nearly complete, but there is still the problem with the login.php…

So the question is:

What can cause the frontende to throw the login.php?
Most said, this is bacause of max queries/user*hour. I analysed the situation with query monitor.

The problem occurs after adding about 20-30 forums (over bbpress). That are about 3000 queries in sum (for showing and adding the forums).
Could that occur also from side of bbpress? Or is only WordPress responsible for showing login.php?

Could it be also the hoster?

What is very strange: If it occurs, it shows the login.php also for guests, that logged not in before.

What I did until now:
– Optimized all tables
– Deleted all inactive Plugins
– Analysed with query monitor and add-on for query monitor (bbpress, buddypress)
– Widended the max queries to 150.000 queries/user*hour without bettering the situation –> I don´t think the reason is max query!

Please give me more detailled information, where the problem could come from.

Thanks very much in forward!!!