I can confirm this as well, happened some minutes ago.

In the netherlands it happens in the evening hours or at night very annoying

This thing gets annoying.. I've couldn't open the forum for few hours!

I believe we now found the reason, or as I should better say, the culprit . More about that later.

more than half hour with a server error answer until right now

Maybe that was the wrong culprit? Forum did not respond for a pretty long time.

It's pretty hard for me to open it even now... Sometimes it doesn't load..

I have a suspicition that if you have your browser clearing history, cookies et al on exit, the site seems to take longer to load the first time you start browser again sometimes returning error.
Once loaded again though, is instantly there even if you leave site and revisit it again as long as browser is not closed.

I'm afraid we have to live with the problem at the moment, as there is no quick solution. But we're working on a solution. -The problem is caused by a person that opens lots of pages and starts many lengthy search processes, using several servers with different IPs. This blocks other users.

Maybe you could add a captcha to the search? Or only offer the search function to registered users.