The message about "reading preview cache" indicates there may be some corruption of the previews, and that might explain the other symptoms as well. First delete the preview cache (it is a folder whose name ends with Previews.LRDATA, in the same folder as your catalog — do not delete .LRCAT catalog file!)
If that doesn't work, my next suspect is that your hard drive is misbehaving and that's why you get these problems. I would definitely perform some diagnostics on the drive, and maybe even have the factory do some checking. Another possibility is to temporarily move the catalog from whatever drive it is on to a different drive and see if the problem goes away.
Side note: photo files on a SSD isn't necessary, you will never notice the speed improvement over photos on a regular HD. Catalog and previews on the SSD is usually a good idea.
Turns out you were on the right track. The enclosure itself with t-bolt connection began to fail giving errors. I switched to USB-3 and things improved somewhat but I continued to hit errors. Turns out the second SSD from the same supplier with the same name is NOT directly compatible with my original drive - this despite a tech's assurance they were. I'm exchanging the enclosure and obtaining a new drive and will put it all back together. One normally doesn't assume hardware failure, so it took me awhile to work it out. As for you aside on the value of storing on SSD, I acknowledge that the improvement is not huge, but given how many images I manage, I judged it to be worth the cost. I'll keep an eye on it and see if it was a frivolous expenditure.
It has been a long, messy and painful process to manually pull my pix off the failing drive, but I've recovered 95% - 98% of the images, but it took many hours of work. Hopefully things will improve.
Thanks for your thoughts,
With regards to putting photos on the SSD, I guess if you have the space, why not? But a recent study showed only trivial speed improvement ... 0.05 seconds per photo ... using an SSD compared to a hard disk. Will an SSD Improve Adobe Lightroom Performance? | Computer Darkroom