Unable to export VM from Hyper-V

The backup and the export have one thing in common: the source. There is probably something wrong with your VHD or other Hyper-V files.

This guy solved it by copying the data inside the VM using Robocopy, but that may not work for you: https://evotec.xyz/hyper-v-incorrect-function-when-trying-to-move-files/

Your updated post shows that your drive is failing. If chkdsk /r can't fix it, you'll need to use a different recovery method. Data loss is a near certainty here. You need to decide if saving this machine is worth it, or if you want to rebuild. No matter what, you need to stop using this disk, even if chkdsk /r can fix it.

A week after having to recover the VM from backups, the physical hard disk was recovered!

This was done by removing it from the server and then it was reattached and reimported and started working again. So worth a try if all else fails.