Please confirm that you are running one of the following versions of Veeam Backup for Microsoft Office 365 before upgrading to 5a:
3.0 (build 3.0.0.422)
4.0 (build 4.0.0.1345)
4a (builds 4.0.0.1553, 4.0.0.1580)
4b (builds 4.0.0.2516, 4.0.0.2549)
4c (builds 4.0.1.519, 4.0.1.531, 4.0.1.545)
You can check this under Help and Support > About in the Veeam Backup for Microsoft Office 365 console. To upgrade from earlier versions, please contact our Customer Support. After upgrading, your build number will change to 5.0.0.1070.
Upgrade from the beta version of Veeam Backup for Microsoft Office 365 5.0 is not supported.
To upgrade from Veeam Backup for Microsoft Office 365 builds 5.0.0.1061 and 5.0.0.1063, please apply Cumulative Patch KB4078.
If you are using Veeam Backup & Replication or Veeam Cloud Connect 9.5 Update 4, 4a, or 4b, install Veeam Backup for Microsoft Office 365 4a. For more information, see KB3035 and KB3089.