In order to comply with the ADIF standards related to location and callsign fields (which is required by TQSL v2.5.2 and later), it is now necessary to populate the "Operator" field in Logbook's My Station. This is your primary callsign; entered without slashes. This is the value that is passed to the DX cluster when you attempt to login.
(Previously, we know that some operators were putting their name in this field. This was not the purpose of this field.)
Within Logbook, go to Tools > Configure > My Station and put your primary callsign (without slashes) into the Operator Callsign field as shown in this example.