Hello,
I'm still evaluating qcad, and am currently working on some competition drawings.
Positive things first:
The respective drawing (a site plan delivered by the city authorities) containes one big solid hatch that is made of hundreds of closed boundaries. While this brought BricsCAD (my main program) to its knees, qcad handles this file with complete ease, congratulations!

But after some hours of work, I stumbled over a quite nasty bug: I tried to draw a polyline in model space, and got a "transaction failed" message (although the current layer was not locked) and then suddenly all entities in the drawing just disappeared. I checked my block definitions, and they too were empty. I therefore saved the drawing under a new name and closed it. However, there was no sign of qcad becoming unstable - it opened the previous version of the file without problems (I restarted the program nonetheless...).

Attached are the empty file (V1-2_corrupted.dwg), the last intact save before (V1-2.dwg), and logs of the command line as well as of the console.

Running audit in BricsCAD on V1-2.dwg just lists 57 errors like this:
Name: AcDbTextStyleTableRecord(6446)
Value: Last height 0
Validation: Not >0
Default value: Set to 2.5

Hope this can give some hints - if not, don't waste time on it, I would come back to it should I encounter such a problem again.