I can't read/speak German but the gist of this message seems familiar.
I can't post comments to my blog anymore because I'm now using an IPv6 address. Looking in the serendipity_comments table in my database I see that the IP column is a character varying(15) which implies to me that there are underlying issues here. Would it be sufficient to change that column type to just a varchar or does the code perform logic on the IP address?
I'm running s9y v1.1. The host is http://blog.etc.gen.nz
which is iPv6 enabled if you need somewhere to try it out.