I have an iMac running OS X Mavericks and OS X Server. The OS X Server services running are Time Machine and Xcode.
A couple of times per week, the iMac gets into a state where I cannot log in. I can select my user account on the login screen, and I can enter my password, but the password field just shakes as if I have entered the wrong password. This happens for each of the three user accounts on the machine.
After rebooting, we can log in for a while (a day or two), and then the can't-login state recurs.
I don't know whether this is related, but sometimes this same iMac gets into a state where everything is really slow. Activity Monitor shows that CPU usage is low, memory pressure is low, disk is not full, but everything seems to be running at 5% normal speed. It's fine after a reboot.
When the iMac is in one of those bad states, I can still ssh to it from another Mac, and I can "sudo shutdown -r now" to restart it.
Any ideas about what is going wrong, or how to diagnose the problem?
After uninstalling an old version of VirtualBox about a week ago, the problem hasn't recurred. Maybe its kexts were causing problems?