Vale's casualty file states the aircraft crashed at vK1053, which according to this site:
http://www.echodelta.net/mbs/eng-translation.php
is
Latitude : 50.84574°
Longitude : 5.03748°
5 miles NE of Tirlemont.
Same file says the aircraft was on a night-fighter sortie to Bremen and Dortmund, no doubt supporting raids on those cities. They took off at 18:07 - not sure what times the raids were.
I've no matching Luftwaffe claim, however it seems another Mosquito reported shooting down a Ju 88 4 miles southeast of Diest, virtually the same place MM678 crashed. Sadly, I'd be willing to bet that "friendly fire" is a possibility.