If you can accept that the AADconnect, is dead in the wather for a short period of time, a simple solution is this.
Use task scheduler:
1. Stop the AADconnect service shortly before the Veeambackup is set to run. (Powershell Stop-Service -DisplayName 'Microsoft Azure AD Sync')
---- The backup is running
2. Start the AADconnect again after a the backup has finished. (mine takes about 15min, so I start the service after 30min.) (Powershell Start-Service -DisplayName 'Microsoft Azure AD Sync')
My backup has been running 3 times this weekend without any further problems