Could be hardware or software problems. Hardware could be local, anywhere along the network route or on the server end. Software could be client application, client OS, client networking, network configuration (in any device along the route), server networking, server OS or server app. For any software component, a configuration element might be incorrect. The account might be invalid, inactive or unauthorized.
There are actually so many possibilities and variations for each one that there is no way to list them. The list is essentially a list of every kind of bug or problem that can happen.