When does it come up? If the only thing is that you can report an error, then you can ignore it. Error reports also capture irrelevant messages.
If this is a pop-up that appears without you trying to report an error, what does it say?

No, you don't want to ignore sync errors. Someone will look at the error report

[JavaScript Error: "Error uploading full text for item NZXSKZIT in [group library]:

ZoteroObjectUploadError: Item 12937678/NZXSKZIT not found in library"]

Can you provide a Debug ID (different from a Report ID) for a sync that triggers this?

Looks like it's a lot of lines though. Maybe because I've been uploading a lot of new documents. Let me know if you need me to run it again after less work.

Actually, I suspect it will.

In your library beginning with "I", paste NZXSKZIT into the search bar in All Fields & Tags mode. That should find an attachment. If you make any change to that attachment, such as adding a temporary tag, does the sync then go through?

If you go to Tools → Developer → Run JavaScript, and run this code, what does it show in the right-hand pane?

return !!(await Zotero.Items.getAsync(12937678))

Can you provide another Debug ID for the sync failure?

Sorry, I gave you the wrong code before. Try this:

return !!(await Zotero.Items.getByLibraryAndKeyAsync(12937678, 'NZXSKZIT'))

Assuming that returns true, which I'm pretty sure it will, the NZXSKZIT item still exists in the "IH" library. It might be in the trash, though.

In any case, this code should select the item:

var item = await Zotero.Items.getByLibraryAndKeyAsync(12937678, 'NZXSKZIT');
return await ZoteroPane.selectItems([item.id]);

If you make any change to that item (e.g., adding a temporary tag), the sync might then go through.

However, I did get a result when I searched for it in the trash

No more sync error, by the way. Thanks for sticking with me and helping me fix it!