one other suspicion is about the capture standard output into messages being overflown with the duplicate torrent--that should explain why the debug version doesn't hang while the non-debug one does--please try turning off View/Options.../General/Options/Capture standard output and restart.
are you sure you're setting LV2_PATH is a way that is visible by qtractor when it runs?
try it with the following command line, literally, for instance:
LV2_PATH=~/.lv2:/usr/local/lib/lv2:/usr/lib/lv2 qtractor
one other suspicion is about the capture standard output into messages being overflown with the duplicate torrent--that should explain why the debug version doesn't hang while the non-debug one does--please try turning off View/Options.../General/Options/Capture standard output and restart.
byee