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While trying to log in to the GP Web Client 2013 some of the users get the error "an unexpected error has occurred. press the sign-in button to reconnect to the application". The Corelation ID is 6555292c-8af7-4464-939c-6e54a4fbf5ba.
The services are all running and the only difference is that these users are in the same forest but in a different domain.
Even when using the same machine some users can login while other get this message. while all users when logging in to the GP desktop client can log in.
When you get an "unexpected error" on the sign on page, you need to check the Dynamics group in the Event Viewer on the server machine. There should be an entry that was logged for the error. It will contain details about what went wrong, and should be helpful in tracking down the issue.
Stop and restart all your services. Also as Steve noted check the Event Viewer and see what it says.
Remove Cache, Temporary internet files from browser, restart the server and then check again.
If the issue still exists, set requireSSL="false" in the file web.xml (C:\Program Files\Microsoft Dynamics\GP Web Client\GPweb\Web.xml).
Please refer the below link