Problem after Ubuntu update - boots to Guest login

Hi everyone, I am hoping a Linux guru will be able to rescue me.

Local box with GTX 1070 running Ubuntu 16.0.4. Video is through the GTX 1070 at 1080p.

After running Software Updater, it boots to the Guest login. Video is turned to a lower resolution.

I now have to go to Advanced Boot Options and select several updates back to boot from 4.15.0-123. Then everything works as normal.

I have done an internet search, but found no one with this exact problem. Many suggest manually disabling Guest mode. Frankly, I am afraid to do any surgery on the OS for fear it will make the situation even worse, e.g., an unusable system

This problem has appeared before. At that time, I waited a month, did a Software Update, and all was repaired by skipping an update. But now that strategy fails now.

Any suggestions?