Thumbnails disappear after running Diagnostics

[BanjoTom]

Once again I've had some thumbnails disappear.  No filters are in operation, and while the thumbnails were visible after I initially indexed them, they disappeared immediately after I ran Diagnostics.  As happened last month, the files are clearly IN the correct folder -- they show up in Windows File Explorer, but are no longer visible in the "Media & Folders" view.  I can find them in the Timeline, and they show up as small images when I highlight the folder in "Media & Folders." 

I created copies of the .jpg files in question and saved them as .bmp and Photoshop .psd images, but those exhibit the same behavior.

I worked with the database with "Full Logging" set under support, and have attached both the log and a screenshot that shows the small preview images that the folder displays, and also shows no filters in operation. 

I'm again quite perplexed.  Suggestions?

[Mario]

There are warnings like

Write-back Cannot write to off-line/read-only file C:\Users\Tom\Desktop\te....
Write-back Cannot write to off-line/read-only file E:\4JoanF....

but these are harmless. If these files are made read-only for a reason. If IMatch should be able to write to these folders, you'll have to check.

ExifTool complains about being unable to access the preview image in some of your files because of unknown or corrupted data. Also just a warning.

Did you keep the diagnosis log file?
If the thumbnails vanished after the diagnosis, where there any errors reported?
The diagnosis may have found some problems in your database and fixed them....this should be logged in the IMATCH_DIAG*.* file.