First of all, thank you for testing development
builds of Tcl so we can catch these incompats
before releasing them.

My suspicion is that tclUdp is breaking an
interface contract (admittedly one not spelled
out all that clearly) and recent Tcl core changes
have exposed that bug.

To confirm, can you please point me to the latest
tclUdp sources so I can reproduce the issue with
my own testing?  Either latest release, or latest
dev state.