I have an old Mac Pro tower (2009; firmware 5,1). I just updated to the latest El Capitan OS X 10.11.5 from 10.11.4. Since the upgrade, the computer has not been displaying screens (I have an Nvidia Quadro K1200 graphics card). However, if I use screen sharing from a different computer I can see the log in screen. However, if I try to log in, I can not. If I type in an incorrect password, the little typing window immediately bounces left/right. If I type the correct password, the typing window disappears, to be replaced by this spinning wheel (shown here in very low quality):
This spinning wheel usually means a successful login. It spins for about 5 seconds and then gets replaced by the login window again.
Any thoughts or suggestions?