Recently I tried to stop a VPG in Zerto after some DR testing, however, the Stop failed and the errors showing where : 

Forcibly delete VPG in Error State : Undo Log Isn’t Empty can’t start new transaction

After lodging a call with Zerto Support, we had to forcibly remove the VPG Via Powershell and Re-Add it back

**Before you do this I recommend you log down all the VM’s in the VPG and the Temp Data Drives

You will need to download and install the Zerto Powershell Tools : Zerto.PS.Commands.Installer for your version

After this is run you need to Open the Zerto Powershell and run 

ForceRemove-ProtectionGroup

Enter the VPG Name without Quotes

Enter the IP of the Zerto Server and username and password ( admin / password ) 

Once removed you can use the Zerto Virtual Replication Diagnostic Tool to import the VPG from the old setting file :

Then navigate to : 

C:\Program Files\Zerto\Zerto Virtual Replication\ExportedSettings

Find the latest backup and try and re-import the VPG

If this fails for whatever reason you will need to use the GUI at the customer side to recreate manually hence Note to backup VM names and Temp Data Drivers