I recreated the backup job for my organization in VBO365 and suddenly two users where missing (including myself) after I clicked on Add - Users on the page "Select objects to back up".
I found the solution on this page: https://itpro-tips.com/2019/usertype-em ... type-vide/ The missing users where created before before 31st August 2014 and because of this the field "UserType" in AzureAD is not "Member" but empty!
After I set the field to "Member" with the command referenced on the linked page the missing users where listed and I was able to add them to the backup job.
Maybe this helps someone
Florian
Loving (and fighting with) Veeam for over a decade!