The Serendipity Anti-Spam plugin allows to utilize the blogg.de IP blacklist service to block spam. Their service seems to have ceased existence, or at least is rejecting connections. This can lead to comments to your serendipity blog to be rejected. You can easily disable the blogg.de blacklist service in your Anti-Spam plugin configuration.
Note that this option is by default disabled in Serendipity since blogg.de announced that they are no longer actively maintaining the blacklist. A well fit alternative to this service is the Akismet API, which the spamblock plugin also supports.