The interesting part is that the comment preview works fine, just the save fails. Probably
this means that the blogspam plugin is the culprit (hence the error in RPC::XML::Client library).
I'm using version 3.20100815~bpo50+. Thanks!
+
+> I've filed an upstream bug about this on RPC::XML:
+> <https://rt.cpan.org/Ticket/Display.html?id=61333>
+>
+> Worked around it in blogspam by decoding. [[done]] --[[Joey]]