When installing the Excel Timetrack Server, a new ODBC connection called WSES10 is created on your IIS server. The server name in this ODBC connection must match the ODBC connection name used by your organization exactly.
More Info: When a login is made in GP, it utilizes the server name in the ODBC connection to encrypt the password (except the "sa" account). For a client to utilize that password in GP or Excel TimeTrack, it must have the same ODBC server name.
In the case of Excel TimeTrack, it utilizes the ODBC WSES10 connection on the IIS server, not the ODBC connection on your local client or remote desktop.
If you have confirmed that the usernames and passwords you are using are correct, you repeatedly get invalid credentials on multiple logins, but you can also log in as "sa", confirm your WSES10 ODBC connection match your other GP ODBC connections in your environment. If it still will not connect, in the second screen of the ODBC setup for WSES10, enter valid credentials to get additional configuration information from the SQL Server, test the connection, and try logging into Excel Timetrack again.