Yesterday I saw the error a second time, so I thought it would be helpful to others to share.
Error happens when VBR tries to read vSphere information for the VM to back up. Error says "Error in the application" like you see in the screenshot.
After some troubleshooting we found the reason for this. It was because TCP-port 902 was not allowed (in all firewalls) from the Veeam proxy server to the ESXi host that was hosting the VM to back up. After the port was enabled in the firewall(s), backup ran without further issues.
- Used ports for VBR see here: https://helpcenter.veeam.com/docs/backup/vsphere/used_ports.html?ver=110
@Cragdooalso shared this error some time ago. In his case the error was caused by a name-resolution problem on a new Linux Veeam proxy server.
I hope this post safe others some time when this error appears.