Hello,
The error message you are encountering indicates that the server you are trying to add to the RDS environment is having problems getting the local database connection string.
You can first check the registry: Open the registry editor (' regedit ') on the server and navigate to:
*'HKEY\_LOCAL\_MACHINE\SYSTEM\CurrentControlSet\Services\Tssdis\Parameters'.*
Verify that the value 'DBConnString' actually exists. In the standard setup for using Windows Internal Database (WID), this key may not exist if the server is not configured properly for Remote Desktop Connection agent High availability or if the registry has been manually changed. If this key is missing, you may need to properly reconfigure the server or restore the registry from the backup (if available).
Also, given that you are using WID, you can reinstall the remote desktop service roles on the server, especially the connection proxy role services. This helps ensure that all components and configurations are reset to their default state, thus resolving any misconfigurations.
Note: Serious problems might occur if you modify the registry incorrectly. Therefore, make sure that you follow these steps carefully. For further protection, back up the registry before you modify it. For more information about how to back up and restore the registry, refer to How to back up and restore the registry in Windows.
I hope this helps.
Regards