@chaley, I maybe find it...
I was wondering why I got the error immediately in a clean installation too. For me it have something to do how calibre is managing the searches. I use saved searches in VL's as well as searches defined by the VL-dialog.
I delete from the test library the files metadata.db and metadata_db_prefs_backup.json to get rid of all database setup, open calibre and told it to restore the database from scratch using its own metadata.opf files. After that the errors gone away, what do not really surprise me (as there are no searches available to provide the function with values. So the field was empty for each book.
Then I start to build up the VL's via create VL's manually again as they was listed in the dialogue for editing VL's. After I put in the forth VL search string the error came back again. (see pic)
I then go back and delete the last VL definition and the third after that too. and the error go away. After checking the search terms I think what's going on here.
I have two searches, one for books in German and one for books in English with following definitions in the test library:
languages:"=Deutsch" or "=German", languages:"=Englisch" or "=English"
After I changed this to
languages:"=Deutsch" or languages:"=German", languages:"=Englisch" or languages:"=English"
the test library works.
What makes me wonder is, why on one system this makes no difference and work fine and on a other systems the function crash and is able to influence other columns with definition "build from other columns" like I had it for the authors sort or calculation columns. In addition I know the old definitions was working for long time good. It seems like there is a little glitch in the chain. Maybe this will help to find out what happen with some installations to find the root cause.
@Kovid,
by doing this test I recognize, it would be nice having the possibility to export/import all or single a definition of Virtual Libraries and saved searches. This will help to use them selective for libraries (e.g. like we have it in the editor). Maybe something you can consider to implement some when in the future. At last for me it is a benefit that I was missing in the past and just remember back again by this action.
