The nohup affects any shell command issued ... no hang up ... complete the script ... period. I use it only when the ssh server timeouts kick in before the backup/restore commands have a chance to complete.
The other thread you referenced dealt with an addon in a course and the poster did find a hack that fixed his issue .... relating to the addon/plugin.
The restore begins with the backup ... so do you have addons/plugins active in the course you are trying to backup? What if you created a backup, via the Moodle Admin UI, set backup preferences to a directory outside of Moodle filedir and excluded anything/everything that was non-core (ie, no addons/plugins)?
On the 3.0 server ... have you run the hidden/still experimental health check?
https://site/admin/tool/health/
That does find issues with Quizzes and does offer SQL suggestions on how to fix. If you find any, run the SQL fix. Any of those Quizzes in the course you are trying to backup via moosh and restore to higher version server?
I would think that using command line moosh, if there were an error it would display or one could setup php to log errors. In one shell window, run the moosh command. In another shell window, run tail -f /var/log/php-error.log ... that shows the error log in realtime.
Afraid there is just not enough info (you don't know what you don't know to share) to know and share exactly what the problem might be.
Check PM in Moodle .... I might have an offer you probably shouldn't refuse but do realize the backup contains users that I would be privy to.
'spirit of sharing', Ken