Thanks for reporting this bug. I think I have diagnosed the problem - this is probably due to a change in behaviour of /sbin/fsck, where it now relies on the PATH environmental variable to find /sbin/fsck.ext3 etc. instead of just searching /sbin and a few other standard places. That is a significant difference from the version of fsck in Jessie.
I'm currently working on a fix for this, which will probably form part of (upstream) release cryptmount-5.2. As a work-around, you could temporarily disable fsck by using "flags=nofsck" in /etc/cryptmount/cmtab.
