Error 500 while logging in

It's since server move that I have noticed such message (500 - internal server error. If you were trying to use search please go back ...) while using a legitimate browser (this).
It happens while logging in, if logged in while replying to posts, and so it makes a lottery doing normal forum actions.

Changing user agent string via browser control panel will avoid all troubles, but it seems to me more a workaround than a solution.

Thanks.