Yes, it's C++. I'm reasonably convinced the problem was a race condition in initialising the GUI; it appears that the debug logger would try to write to a text field that might or might not exist, probably because a different thread was creating it. (I don't have full control of the threading since I'm using Qt for my GUI stuff, so exactly when my code is called is unfortunately a bit black-box-ish.) I believe I resolved the issue by introducing a special debug stream for use during startup, which doesn't try to write to the GUI but instead writes to a log file.