Will contact you via PM ... other info/discussion related (maybe).
At the bottom of what you've shared it says 'Service not available' - a 503 error and does suggest one of two reasons ... server set to maintenance mode or 'capacity problem'. Another admin user ... or something scripted ... could have put the site into the maintenace mode. You'll have to check apache logs and other logs in Moodle to see if that were true or not. Multiple admins always has potential issues cause you can't see them and they are not informed of another admin level user doing something/anything on the system in real time.
The other item 'capacity problem' could be a space problem ... Moodle doesn't have any tool for you to see if there is a space left where moodledata resides.
Just so I understand ... you were attempting to restore a course from a 2.3.3 version of Moodle to a 3.0.highest? Is that correct?
The basic process for restore .... .mbz file is uploaded (or acquired from a repo), it is then un-compressed (un-tarred/gzipped) in at temporary directory located in moodledata/temp/backup/ ... that a8ca .... _bunch_of_letters_numbers directory. At the same time Moodle opens a log file by the same name ... a8ca...dotlog. When moodle steps through it's restore process it records what plan/stage it was executing to that .log file. In your error, that's what was un-available - that a8ca...log file.
Moodle 3 now has a task list ... that's tied to cron jobs .. one of them is to 'clean up' temp areas of the data directory and tables related to things like backups/restores.
Search for 'task' in Site Admin menu and see if there is something turned on that might have kicked in when you were manually restoring a course.
A sure sign that disk is full ... users can't login. Moodle now defaults to using 'sessions' in files rather than DB. If disk where moodledata resides is full that is also where one finds the 'sessions' directory to store those small session files. No space. Can't write to sessions ... can't login.
But, one could have enough space left where moodledata resides to begin the restore process, but after the .mbz file is un-compressed and the restore process begins ... opps! No space left.
Only way to know for sure would require access to the server out side of Moodle ... ssh sorta thing.
Sooooo ... not enough info to say for sure.
'spirit of sharing', Ken