Hard for anyone to give you a solid answer considering the situation ... 500 errors don't tell anyone much. And even though you've shared server setup info, no one has the backup file you are working with.
So a company created the backup for you to restore. Correct? Then follow the entire chain/path to the problem. START with the backup file. Change the .mbz extension to a .zip and extract the zip file to a folder on local machine. In the extracted files find the moodle_backup.xml file and open it with NotePad. In there will see information concerning the backup ... including the version of Moodle the company was using to create the backups ... as well as the URL to that Moodle instance.
IMHO, if that shows an earlier version of Moodle, like a 2.5.x or a 2.4 or even a 2.6.x I'd contact the company and request a version of backup that matches your instance. One would assume a company selling such products would test their backups so they could be informed and be alerted to any problems that they might be able to resolve for the customer prior to selling the product ... uhhhh, isn't that called 'quality assurance'?
You might get the product FREE of charge should you help trouble shoot this for them. ;)
On your site: turn on debugging. Check the data directory: moodledata/temp/backup/. Are there any .log files there larger than 0 bytes? If so, open with NotePade and inspect. They may not tell you specifically the issue, but it might provide clues. If you find any reason to tweak settings ... IIS, PHP, MySQL other whatever, then TIA (try it again).
Check server logs - php, mysql, IIS. ie, the normal things one would do in a failed restore.
My 2 cents, of course!
'spirit of sharing', Ken