Veeam just sends requests to the hypervisor in order to create or remove a checkpoint, therefore I assume there is an issue at the hypervisor level. I would recommend to review Windows event logs on the Hyper-V host, probably you would find there some useful information. Also, you may contact our support team and let our engineers to help you with troubleshooting as our possibility to analyze technical issues on forum is very limited. Please don't forget to share a support case ID with us.