App crashes with system error

[Sterch ltd.]

I've got the following error.

  File "/path-to-eggs/ZODB3-3.10.2-py2.6-freebsd-8.0-RELEA
SE-p1-amd64.egg/ZODB/serialize.py", line 422, in serialize
    return self._dump(meta, obj.__getstate__())
SystemError: NULL object passed to Py_BuildValue

Further debugging revealed OOBTree object with incorrect state.
App is Bluebream/ZTK based. It had worked stable for a long time before crashed with the error above.
OS: FreeBSD 8.0, Ubintu 10.10
Python: 2.6