I have started receiving quota exceeded errors for one user too, and the user definitely has no quota restrictions (neither does the group).
The debug output shows interesting things:
/bin/sh: line 1: let: HD_QUOTA=*1024: syntax error: operand expected (error token is "*1024")
Apparently procmail thinks QUOTA is set and then tries to do arithmetics on an empty expression which obviously goes wrong.
I didn't dig into procmail yet but maybe this helps someone figure out the real problem.