Hello! Thanks for your suggestions, Rene. It made more sense to continue here instead of on the Shout Box.
To recap, MAME 0.171 is crashing when I load any system/game/softlist item from QMC2 0.61 (all on Win 7 64bit).
My Working Directory has always been empty. setting it to the MAME64.exe location didn't make any difference.
In case it's relevant, I make sure to overwrite the old MAME and QMC2 files when moving to a new version.
Lastly, here's an example from the 'Front end log':
18:14:19.567: starting emulator #0, command = E:\emu\MAME\mame64.exe -bloom_overdrive 1.000000,1.000000,1.000000 -mouse -joystick_deadzone -0.900000 -joystick_saturation 0.250000 -autoframeskip -multithreading -rompath "E:\emu\+Images\ROMS;E:\emu\+Images\Software_Lists" -hashpath "E:\emu\MAME\hash" -artpath "E:\emu\MAME\artwork" -numscreens 3 hp48sx
18:14:19.762: emulator #0 started, PID = 258412832, running emulators = 1
18:14:59.510: emulator #0 finished, exit code = 255 (unknown error), exit status = normal, remaining emulators = 0