Hi @GeoJosh - I am experiencing the exact same error in trying to register both my host site and my federated ArcGIS Server sites with a new Monitor 2023.3.1 setup on a Windows Server 2022 VM.
I am using the host name https://myserver.net:6443/arcgis and the ArcGIS Server primary site account and password, per the Monitor help instructions.
As with @Feike , the portal component registered just fine.
Everything (Portal, Host, Federated Site) is at 11.1 on Windows Server 2022 VMs. In fact, the 11.1 installs on these servers are new, not upgrades as we migrated servers from Windows 2016 to Windows 2022.
The Usage Reports from the ArcGIS Server Admin Directory:
As you can see, these are the default usage reports, I do not see any reports with 'number-only names'.
Any suggestions and help is appreciated.
@DerekLaw