We were unable to identify the full root cause in this case. We were able to identify that there was a reported issue with the integration between SDL TMS and Directory Services. There was no evidence of any error in the log files to indicate the root cause.
Although we have not been able to identify the root cause of this issue, a subsequent restart of the server restored/resolved the connection issue.
If this does not help, please submit a Support case as explained in this article: