Qtractor hangs when MIDI Master Out is connected to MIDI Master In.
 

Only sometimes, but I have been able to reproduce it about 80% of the time.
In the "Connections" Dialog; I make the following connection:
The console output at the bottom of the screen starts scrolling with
a repeating message that looks like:
23:00:00.500 CTL: MIDI Channel 0, value, blah blah....

To me this looks like a "midi feedback loop" in which a controller value
is "stuck" I can see Controllers 7 and 10 identified value is 128.

Why would anybody do this intentionally? they wouldn't but it happened to me by accident when I was connecting up ZynAddSubFX and clicked the wrong
thing. It does not happen when there are no tracks created in the editing window (ie. first open Qtractor with a blank session).

The only way to exit is to force quit, so be careful when making connections, and as always, be sure to save frequently.

I am using the version of Qtractor packaged for Debian 6.06:

Version: 0.4.6
Build: Jul 22 2010 21:32:46
Debugging option enabled.