check Regenerate if view/sheet is not up-to-date. This will create a thumbnail for the non-opened view to be included with the initial backup version and eliminate the error message when saving the backup version.
Cannot Restore Backup Version an Unknown Error Occurred - The Revit Clinic
However - if you have already come across this problem and it is too late to check the above box ... you may be able to get the Restore to work IF you reinstate an appropriately named preview.xxxx.dat file into the _backup folder. Its worth a try if you are struggling to recover some important data.