OK, I managed to find a solution (thanks to Tomasz Mrugalski for the help). Tomasz was supposed to answer to this bug report, but since he didn't answer yet I'm going to summarize the info he provided to me.
As far as I can see (didn't do any extensive testing, just configured and deconfigured one client), dibbler 0.6.0 RC2 downloaded from http://klub.com.pl/dhcpv6/debian/ worked (I downloaded the server i386 deb). Also, the condition is due to a bug in dibbler (found in 0.5.0, but was there probably since some earlier version). It parses one option in the wrong way (as 4 bytes instead of 2), but it's bug-to-bug compatible with the client, so you only notice this if the client and server implementations are different. http://klub.com.pl/cgi-bin/bugzilla/show_bug.cgi?id=127
