An error while trying to establish a connection to the Exchange server

Hello,
My Internet connection is OK, I don’t have any error while surfing on the Internet and no lag but I have this error when I try to establish a connection with Exchange server. It worked just fine until now and I don’t know how to fix it. Can anyone help me please to solve this ?
Thank you !

ExFolders
An error while trying to establish a connection to the Exchange server.
Exception: The Active Directory user wasn’t found.
OK
