Thanks, that eventually got me working again.
Since I have standalone LR6, the only part of that fix that I could do was to change the name of C:\Program Files (x86)\Common Files\Adobe\SLCache, which I did.
When I then restarted LR6, it gave an error code and told me to uninstall and reinstall, so I re-downloaded and did that.
Now the program works again, but it still has a "Lightroom mobile trial expired" message at the upper left. Does this mean that I'll have the same problem in a few weeks? I tried changing the identity plate, but that didn't get rid of the message.
I had purchased a standalone version upgrade. When I installed it (and reinstalled it), it prompted me for the serial number and I entered it (and, since it was an upgrade, the serial number of my previous version). My splash page says Lightroom 6.
That's how the standalone versions have always installed. I wasn't aware I was converting anything.
But it still says "Lightroom mobile trial expired".