Not sure if this is going to help but we did run into a similar issue when restoring a backup and receiving a permission denied error. It was on 10.3 though and we were using a VM. The only way we were able to 'fix' it was by doing a reboot of the server (which we did after troubleshooting every other issue we could think of; folder security, drive security, user permissions, etc.). After rebooting we did not receive the Permission Denied error. and the restore completed. The underlying issue was we lost remote connectivity briefly during the install which we believe hampered the drives permissions, but we were only able to find limited information in the logs (basically Permission Denied on the folders).
Again, not sure if this will help since it is one of those standard questions (Did you restart the computer), but since the error was on the same line on the CMD I figured I would pass on what we ran into.