How the hell did you manage to narrow it down to that interrupt?
It took a while... and considering that gdb didn't help much, only indicating what was actually locked, not what was causing it :-p
You mention background threads... How would they work?
Dunno ;-)
But using signals seems kind of hard-core to me. I'd have thought you could just as well, run some kind of looping thread instead to catch the events, which doesn't employ signals. It seems that some external functions expect to be in control of signals when they kick off, and maybe get into a "race" with Blitz's low level implementation.
Don't ask me though. I ain't the expert.