It's inconsistent, but sometimes I get the "unexpected error" message. I restart my computer and then everything loads just fine. It's just annoying to have to reboot the computer to get Lightroom to load. My photo files are stored on an external drive with over 5TB available, and my computer has 1.76TB available.
First, maybe you have done this already, but you should be making regular and automated backups of your catalog file, and these should be stored on a different physical disk than the working catalog. When you create the backups, make sure you have the "Test Integrity..." option checked.
It is my opinion that "unexpected error" message can be caused by an intermittently malfunctioning hard disk or intermittently malfunctioning memory chip. Thus, I would definitely run diagnostics on your hardware.