Make sure the Tableau Server account have permission to access the file.
You can check this in a Command Prompt on the Tableau Server.
net use * \\ExcelFileServer\sharename [tableauServiceAccountPassword] /USER:domain\tableauServiceAccountUsername
Michel -- We had IT handle this yesterday. They said they had...
I contacted them today and asked them to re-do permissions for the Server account. They re-jiggled the handle, and now it's working.