@sialaser
1. Do you cohost the Office 2010 KMS host and the Office 2019 KMS host on the same system?
If yes, what's the version of this system? And what version of system on clients?
Please reinstall "Microsoft Office 2019 Volume License Pack" to check this issue.
2. Besides, I suggest you let KMS clients connect to the specific KMS host via running commands.
cscript ospp.vbs /sethst:<kmstest.contoso.com>
cscript ospp.vbs /act
Then go to KMS host run the following commadn check whether the results of "Key Management Service cumulative requests received from clients" changed.
cscript slmgr.vbs /dlv 70512334-47B4-44DB-A233-BE5EA33B914C
If the "requests count" does not change, it is recommended to disable and re-enable DNS Host to check this issue.
(Please note, if you need to modify registry, please back up the registry for restoration in case problems occur.)
3. Sometimes the error message could have occurred because of the time and date sync issue on the system.
Please check system time and date on host system and clients system.
4. It's recommened to use diagnostic tools from "KMS activation issues", to check the host, and client separately.
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