There is most likely a conflict between *.dll versions. Go to the folder where Veeam Backup & Replication is installed and make sure all *.dll files have the same version:
The conflict can happen if the hotfix was previously installed over the incorrect version, and new methods from the new version are not recognized in the earlier version.
To address the issue, follow the tasks below:
Back-up VeeamBackup SQL database. To be on the safe side, you can use Management Studio.