"Your attempt to log in to the server failed because an unknown error occurred when the user ID and password were being verified. Attempt to log in again."
Resolution:
In our scenario, we had the GP Server on one domain and the end user/client on another. Thanks to Derek Albaugh's comment about ODBC, I quickly launched the application and it failed the test with the following "The client and server cannot communicate, because they do not possess a common algorithm." - I immediately remembered that I had deployed a policy to only allow TLS 1.2 on the end user/client domain. Temporarily disabling the TLS restriction policy and cleaning out the rules in the registry on the client PC resolved our issue.
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