Users can often not log in - username or password invalid

For about 2-3 weeks now it's increasingly difficult to log in to SDP. Even though a correct username and password was entered, the logon-screen tells the user that the username or password are incorrect. This applies at least for several normal users, and I, too, as admin have trouble logging in. I have then updated to 14.3 Build 14302, but it doesn't help. Sometimes the user can log in from an incognito tab of chrome, sometimes this doesn't work either. I have tried deleting all cookies, but that doesn't help. Neither does clearing the site data in chrome developer tools.

Has anyone experienced the same problem and can help getting this sorted out? The program is of little use if users can only log in after extensive fiddling with it... ;-)