Ls Classic 10.4 error using Publishing service

When publishing a collection set I always receive many pairs of error messages, the first is:

readMetadata: Error when Lightroom attempts to process a catalog master image file: “dng_error_file_not_found” (look at the part after “dng_error” for a clue to the problem)

This is followed a few seconds later with a list of files headed with:

Some export operations were not performed
An unknown error occurred. This photo was not rendered. (nn)

Where nn is number of files listed, typically a number in the range 10 to 60

I have checked some of the listed files and all those checked do exist on disk as .dng with .jpg sidecar, in the Lr catalog with all expected attributes.  However, they were not exported to disk as requested (just as the error message says) but others in the same collection were.

When I export these files manually from Lr, everything proceeds perfectly normally.

It would be good to know why these files cause problems to the Collection Publisher so that
a) they are exported as requested and
b) the whole process completes without my involvement

[AMD Ryzen 7 8-core CPU, 32GB RAM, Win10 Pro v21H1 on SSD, photos on HDD. Lr v10.4]