Guest agent at <...> is already in use by VDI Agent at <...>

Symptoms

vdiagent.log has errors like this:
[E 0B/00000011/T0BEC/P0F3C] 03-09-21 15:48:42 - Guest agent at aaa.bbb.xxx.xxx is already in use by VDI Agent at ccc.ddd.yyy.yyy.

Cause

There is more than one RAS farm connected to an instance of vCenter.

Resolution

There are two possible solutions:

1. (recommended) Use a dedicated resource pool for each RAS farm;
2. Configure 2XVDIAgent registry key for VMs and templates from both farms: https://kb.parallels.com/en/124157