The Partner Center Service Health dashboard has a relevant notice on it:

Last updated:December 10, 2024 at 5:33:04 PM

Some Partner users are unable to access customers M365 Admin Center Portal via GDAP (Admin on Behalf Of). Partners will receive a generic permissions error. M365 Admin Center Engineering teams are working on the resolving the issue.

While not an exact description of the issue I believe this is the cause.

https://partner.microsoft.com/dashboard/v2/support/service-health-status