Thanks for the info that storage limits don't affect metadata, and the reference to the kb. I have now disabled automatic syncing, and tried a manual sync instead, and this gives a red error message:
The name 'Democratization, and the World Economic Crisis. " P...' [sic] is too long to sync. Search for the item with this name and shorten it...'

However, there is no item with such a name either on this PC, or on the server, or on the laptop which I also use - neither in My Library, nor its trash, nor in any Group Library or trash.
So this presumably explains the failure to update: but gives me another problem, how to get rid of an item which does not exist anywhere.

Note that "name" refers to someone listed as a creator, not the title of the items.
Hoave you searched for part of this in your various libraries? Obviously Zotero isn't inventing those words, so they do exist somewhere.

@adamsmith This confusion happens a lot. Maybe the message should more clearly say "Creator name"?

Should I try a total reset, 'Restore to Zotero server'? NO!

Don't do that unless specifically instructed to do so.

Yup, I have done that, a number of times, both locally and on server. The only hits did not have the rest of the phrase nor come from the relevant journal.

You said that you examined your local and server trash. Please try emptying both -- including that of any groups.

One more thing, search for the author name, Luckhurst or search journal title 'politics & policy'. (I remember reading that editorial and found it in my library.) You might also try a title search for: "Introduction to the special issue"

No need for a Debug ID. Paste "J5BFZJI5" into the Zotero search bar in the root of your library.

A more general question: I understand why invalid items can't be synced, but is it possible to just put them in a separte queue of 'unsyncables', while allowing all the valid stuff to be synced?

that will be the case with Zotero 5.0, the way I understand the new sync.

J5BFZJI5 is the reference that's actually throwing the error, so it _has_ to be there. Make sure that you
a) have "My Library" selected in the left-hand panel and
b) definitely have your Zotero trash emptied.

that will be the case with Zotero 5.0, the way I understand the new sync

Yes.

could the data entry rules of Zotero prevent the creation of an item with a field which is too long to be synced?

They could, but only by throwing out data, which we don't like to do. An alternative might be to automatically move that data into a child note on import. (Notes have length limits too, but they're higher.)

Zotero is a bit more helpful when you try to sync too-long text in a couple places: for tags, a wizard pops up that helps you edit/split/delete the tag, and for some fields the error message includes a button that takes you straight to the affected item. I think in 5.0 it should be possible to do the latter for creator names too (which is difficult in 4.0 for technical reasons), but I can't remember if that's implemented yet in the current 5.0 beta.

It's true that the data is often/usually junk, but Zotero data does come from many different sources, and the concern is that there might be valid data that made it into the wrong field that you might want to manually move elsewhere, fields that were incorrectly concatenated, etc. But I do think storing it in a note might make sense.