we had a nearly same problem.
Error: Failed to call RPC function 'TestCompatible': Error code: 0x80070583. Cannot initialize COM runtime.
here is the solution from Veeam support
"It is much pretty common with Update 3 when 3rd party monitoring tools proadcastrsVeeam with PowerShell queries and it affects our backups.
I would recommend the following steps:
1) Disable zabbix service
2) Restart Veeam server
3) Make sure that service is disabled
4) Open Veeam and re-try the job
You can use SNMP instead of Powershell as a workaround or refer to 3rd party software in order to have latest updates
Please keep me updated. I will be happy to help or answer your questions. Have a great day!
After disabling Zabbix agent, backups works normally. Do you have some monitoring tools there?