Investigated my graphics driver, in case the driver needed upgrading, but according to the Intel website, all I need do is keep the OS updated, which it is. I can't disable the graphics driver using Lightroom preferences, because the program won't open.
If Lightroom won't start you can disable the GPU support by editing the appropriatre plist - file directly.
Take a look here Adobe Lightroom GPU Troubleshooting and FAQ and follow the steps desribed under section "Solution 2"
I think I have resolved the issue (or at least understand the problem).
I just left LR Classic CC with the beachball spinning, and it flashed up a message entitled Lightroom Catalog Upgrade, that "We need to upgrade your catalog fir use with Lightroom Classic CC; Select Upgrade below to begin. Lightroom will create a new catalog file in the destination below. The previews will be moved to this location and converted for use in Lightroom Classic CC.
The file was XXXX/Lightroom Catalog-2.lrcat.
I selected UPGRADE, and Lightroom opened. It had no files in it, but synched the ~5K I had in the cloud. All good, except that that was all I had.
So, I went into Preferences, General and regarding default catalog, asked for "Prompt me when starting Lightroom". When I selected my original catalog, the program hung again.
It seems as if my original catalog (made when the program first experienced issues) is corrupted. I went into Backups, unzipped the previous catalog file, copied it to Desktop, and used that one to open Lightroom.
And Voilà. All seems to be OK now, with the photos from the Creative Cloud just synching.
Fingers crossed, all is well now.
I tried that, but as I said in 4., I couldn't find the right string.
If the string isn't in the plist file you have to enter the line
useAutoBahn = false
The specific GPU is not supported for GPU acceleration in Lightroom Classic CC, so checking if it is on or off is unnecessary, Lightroom will not use it.
Please state the version number of your operating system.
It seems you have perhaps opened the wrong catalog file. What happens if you find the proper catalog on your hard disk by using the operating system search feature to search for files whose name ends with .LRCAT, and then once found, double-click on it?
So, at the spot where I find "useAutoBahn", I add "= false" ?
macOS High Sierra 10.13.6
I investigated the .lrcat file(s), as outlined in my second post. The last one was made at the time program first experienced difficulties. I backup the catalog every time I close LR down. If I double click on this particular .lrcat file, the program hangs. If I use the second last .lrcat backed up file, I can open the program. Hence my thought that the latest .lrcat file was corrupted i some way. Perhaps it did not back up properly on closing. Who knows.