Since a few days I have a high ping (~200) to the RBI CTF server.
It's only this server for me and on this server most others have a normal ping.

So it seems that it's neither the fault of the server nor mine.

What can be the reason for this?

There was some talking about that problem on IRC yesterday, so you are not the only one that is affected.
I can not remember who it was but they found out that there is a high ping server (the "Deutsches Forschungs Netz" server) in the routing chain to the RBI server. As different ISPs use different routes not all people are affected. Good news is that this should only be a temporary problem, bad news is that you can do nothing about it but wait :/