SO helpful: thanks. Well, it turns out that it was only the first few tabs (from the top) that were empty in that way. They don't seem recoverable but, as you say, the others were all fine as far as I can see. I've deleted them and will rebuild the contents if and when I find I need it! Logs showed nothing because, I guess, nothing loggable was wrong! I appear to have synchronization turned on for some tabs but not mapped to any existing directories. Is there a way to delete/clear all existing synchronisations and start over? I think that's morphing into a different point so I'll submit it separately.
"Backup on exit" is wonderful. I have tweaked it to export to my NextCloud ("Nextcloud" without the capital "C" in the directory structure) so that puts the export cpq file available across all my NextCloud machines for the future. I take it that that is an export file ... which again maps to my next question so I'll just stop this here with many, many thanks (and apologies for the earlier misspelling of "wrecked" as "wreaked": I think that was Freudian as whatever caused that had wreaked so much havoc!)
Brilliant program, brilliant support. Thanks again,
Chris