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 184.108.40.2062)
4.0 (build 220.127.116.115)
4a (builds 18.104.22.1683, 22.214.171.1240)
4b (builds 126.96.36.1996, 188.8.131.529)
4c (builds 184.108.40.2069, 220.127.116.111, 18.104.22.1685)
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 22.214.171.1240.
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 126.96.36.1991 and 188.8.131.523, please apply Cumulative Patch KB4078.