Just for future reference, I had this issue with a customer today.
It seems two items helped solve the issue:
#1 The user, connecting from Veeam to SCVMM had correct permissions in SCVMM and on the hosts. However it was missing permissions on the Hyper-V cluster itself.
#2 We changed the user format in Veeam, for the user connecting to SCVMM, from domain\user to firstname.lastname@example.org
Some log dump for easier search:
Error [hypvcls.domain.local] Disks and volumes discovery failed Error: Failed to determine SCVMM Server version:
The target principal name is incorrect
A call to SSPI failed, see inner exception.
You cannot access VMM management server scvmm02.domain.local.
Contact the Virtual Machine Manager administrator to verify that your account is a member of a valid user role and then try the operation again.
You cannot access VMM management server scvmm02.domain.local. (Error ID: 1604)