The issue was caused by a fileserver which hosts user profiles. The server randomly lost connection with the wider environment. As a result some people could successfully login, no login or get partial access f.e. access to the hosted desktop but e-mail did not work. Other similar fileservers did not have this issue. It is not clear why this server lost connection.
Following a restart of the server at 11:25 CET the server was fully operational again. As some user profiles were still active new sessions could not be established at first attempt. A second attempt was usually sufficient. To ensure no roaming profiles were remaining the hosted desktop servers were all restarted overnight.
We monitor the impacted fileserver closely.