Somebody there who can point out what's the solution for this error while backing up O365 Groups Mailboxes:
Processing mailbox something@domain.com failed with error: Failed to access mailbox.. Access is denied. Check credentials and try again., User '/o=ExchangeLabs/ou=Exchange Administrative Group (FYDIBOHF23SPDLT)/cn=Recipients/cn=user1a4dc7bb' not authorized to access group 'something@domain.com'. Reason: '(0x4D10)'.
Normal Mailboxes, Sharepoint Sites and OneDrives are backing up without problems.
Without seeing your logs this will be a guesswork. Does it happen within a hybrid or Online scenario? what type of a group is it? and so on.
Can you please open a support case and post its ID here, so that we could track the investigation? Thanks!
From what I can see, the user that you use to connect to the office 365 environment has not access to that specific group and can't enumerate the data.