A while ago, using OS X Mavericks, I installed an OS X utility named Proxinator, which was supposed to allow me to lock and unlock my Mac by proximity sensing my iPhone using bluetooth and a companion iPhone app. It didn't work well, but I left it installed.
Recently, and after installing Yosemite, there came a security update which prevented me from logging in. The symptoms were basically the same as described in this question, (black screen, flashing cursor instead of login dialog). The error logs pointed to
/Library/Security/SecurityAgentPlugins/proxinator.bundle. Removing it didn't help, and I was eventually forced to restore a backup that took me back to Mavericks.
Then I tried removing the bundle from the
SecurityAgentPlugins directory, and I got the same problem with the black screen. Restoring it fixed the problem. So something in the system configuration is evidently expecting a plugin that works, but I can't figure out what.
How can I get rid of a Security Agent Plugin?