Hello Jens,
Did you re-compile completely after disabling threads? (Remove all .fasl files -- this is a compile-time option.) Note that this will mean that connection pools are no longer thread-safe, which might be a problem in some applications. The problem you are seeing does seem to be related to bordeaux-threads... but you could check this by making a small dummy project which uses bordeaux-threads but not postmodern, and seeing if you can build a stand-alone app from that.
Cheers, Marijn