I found this old thread about this topic.
https://social.technet.microsoft.com/forums/windowsserver/en-US/30bbc245-4170-44e9-8c96-0c9239a4e61c/exportimport-vm-on-hyperv-with-error-general-access-denied-error-0x80070005
I ran into this issue today when trying to export from a domain joined Hyper-V 2019 host to same domain joined Win 10 machine. Same user on both system, I added the computer account to the NTFS and the sharepoint permissions - and it wouldn't work.
Additional searches around the web lead me to Kerberos Delegation.
Opened ADUC > found the original Hyper-V server > Delegation tab > add new > Win10 machine/cifs
Now my export is working as desired