Afraid that doesn't provide enough information.
The backups come from what version of Moodle?
If un-known, you can extract the .mbz file and open the moodle_backup.xml file with a text editor to see that info.
Also, check your moodledata/temp/backup/ directory.
Log files that are larger then 0 bytes indicate failed restores and probably contain the same info you were seeing on the GUI restore process.
In that directory, there could be directories with very long character/number names. Via something to explore files, go into one of those and see if you can find a moodle_backup.xml file ... same thing with it ... can open with text editor to see the overall contents of the backup file and other info ... like version of moodle.
Other things ... turn on debugging and try again to see if Moodle reports anything.
Also check web service error logs for clues. If the courses are large (or heavy processing), could be you need to increase max_allowed_packet settings in DB config. OR ... increase php setting for memory a script can use, time to run, or other.