Time Machine restore failing with Unrecoverable error 'builtin:prelogin'

Hello,

Trying to restore from Time Machinebackup on a Western Digital (My Passport for Mac) external hard drive gives me this error "Unrecoverable error SecurityAgent was unable to create requested mechanism builtin:prelogin". It would usually fail after about an hour, and from this page I can do nothing except hard power down the iMac. I did find a similar thread suggesting to delete the 'auth.db' file, which I tried and it did get to the end of the restore but then failed with the same error. Any suggestions please?

For background information, my problems started after upgrading to Big Sur where I couldn't open Firefox or Safari, everything was completely slow and basically unusable. I followed Apple KB to restore from backup and got the error above. From last Sunday I've raised 6 cases in 5 days with Apple (senior) support, erasing hard drives, repairing Fusion Drives, reinstalling High Sierra and upgrading to Catalina and trying to use Migration Assistant to get my backed up data onto the iMac. The last call I had said all I could do was manually drag and drop from the backup drive, which was fine for 'Documents' and 'Downloads', but completely fails when trying to import Photos library. Thank you.