We had a close call restoring this site, the backup was on a hard drive in a computer that didn't work anymore.
The backup I thought was the latest version was actually corrupt. And while talking to
@Mikey I actually came to the realisation that it might be on the old system drive. I took it out of there, connected it over usb, took over the directory permissions and found the backup that was actually the last one, AND it was not corrupt. #phew