This is almost hilarious (if I hadn't wasted so much time before I accidently saw this).
Something (I don't have any idea on what) seems to have messed with the Defender settings.
"Protected memory access" (translated from norwegian so sorry if not exact) was removed for:
- "rstrui.exe" on "\Device\HarddiskVolume2" (that is the C: drive with "System Restore")
- "SystemPropertiesProtection.exe" on "\Device\HarddiskVolume2"
As soon as I added them to the trusted applications System Restore worked again.