I have no issues logging into these forums on my work computer (I work for an engineering company). However, at home, repeated attempts within the last week to log into these forums using devices at home have been unsuccessful. One is a Samsung tablet. Also, when logging in from home, I get one or the other of two different log-in screens. One will not accept my userid and password, which clearly work when logging in from work. The other login screen will not allow me to enter anything at all.
I am able to log in at work using Win 7 and IE 11, and at home Win 10 and Chrome, and I just tried my Samsung Galaxy Prime smartphone with Android and Chrome - no problems logging in.
Have you done any recent updates (or not done)? My iphone stopped working on the forums about a year and a half ago (I did not upgrade the software in 2012 when Apple changed how their OS worked).
I had the same problem after a recent update on my iOS devices. It appears the login requires cookies to authenticate, so you need to go into settings and enable cookies. With iOS there is an option to enable them only from sites you visit. No doubt Android and others have similar options.