Qualification for this response ... don't run Windows so have not seen that exact error + you asked for 'ideas' ... soooooooo ....
There's something in those 2 specific courses (obviously), but what?
Check moodledata/temp/backup/ folder for 'hashnamed'.log files. 0 byte .log files are successful backups and any .log file with more than 0 bytes indicates issues with associated course. Open .log files with NotePad and see what they say. They will provide hints as to module/activity/etc.
IF you see any 'hashnamed' folders by the same name as the .log files those are the temporary folders Moodle uses to build the backup .mbz files. Inspection of those folders probably wouldn't hurt.
BTW, they can be safely removed from the operating system level.
Also, as your 2 has grown, data has grown so one might need to tweak mysql settings for max_allowed_packets.
If that is the problem one might see references in mysql logs or web error logs
MySQL server's default max_allowed_packet value is 1MB.
See: https://dev.mysql.com/doc/refman/5.1/en/packet-too-large.html
'spirit of sharing', Ken