It looks like you somehow ended up with a linked-URL attachment that's a child of another attachment — possibly from something one of your plugins did.

Go to the library root and search for WFFFXUUA in All Fields & Tags mode, delete the attachment that shows up, and empty the trash.

If you can figure out steps to reproduce this with a new item, let us know.

You’ll need to follow my instructions above, using the 8-character item key you’re seeing now.

As I say, it has to be the item key you're seeing now in the error message. Whatever caused this — probably a plugin — may have affected multiple items.

Is the search bar in All Fields & Tags mode?

The matching item is the one in black, not gray — the note in this case. That's the parent item. Another item, HJWJU3C8, is the linked-URL attachment that's somehow set as a child of the note, which shouldn't be possible. Same for EUE5GJ7K being a child of 6FFMBRE3. (The linked-URL attachments may not show up when you search for them, because they're not supposed to exist in that state.)

If you check your database integrity from the Advanced → Files and Folders pane of the Zotero preferences, it should offer to fix your database, and it should turn these invalid child attachments into standalone attachments that you can delete or move to an appropriate parent item.

If you have any idea how this happened — again, probably from some plugin you have installed — we'd be interested to know.

OK, yeah, Zotero didn't used to prevent setting a note or attachment as the parent item of an attachment (since it's not something Zotero itself ever does), but we fixed that in February of this year, so in current Zotero versions that Zutilo option doesn't do anything other than log an error.