No, nothing to do with multiple documents.

The "You have modified this citation since Zotero generated it" means you're editing the citation in the document (which you should almost never do), or Track Changes is interfering (though Zotero tries to prevent that). You should be able to get an idea from what it shows for the two versions. Say "No" and avoid editing the citation. If you've been doing this on purpose, see Customizing Cites for the proper ways to do this.

For the other, see Troubleshooting Errors in Word Processor Documents.

It'd only be a problem if you didn't delete the entire field.

But again, the dialog should give you more info. What does it say for the original and modified text?

I accepted any changes and then turned it off. The problem persisted though.

Well, what persisted, exactly? Any citations that were already corrupted would trigger the error the next time you interacted with the plugin. If you got the error about citations added after that point, it would be unrelated.

After a bit of trouble shooting and looking more closely, I'm starting to wonder again if it is somehow related to having two word documents open at the same time. I had an old document with numbered, in text, zotero citations open alongside a new document that I was adding number zotero citations to. These documents were side by side in "Full Screen" view on my Mac.

The old document was open to the bibliography so I could the name of the references. In the new document, I would have the cursor in the location where I wanted to add the citation, click on (or use a shortcut for) the "Add/Edit Citation" button. The Zotero window would then open on my Desktop view, taking me away from the Word documents, I would then swipe back to documents, look at the name of the reference(s) I was interested in the old document, swipe back to the desktop, search for those documents in the Zotero window, click on the appropriate ones, hit Enter, and then it would take me back to the Word documents.It is at this point it would often (but not always) give me one of the dialogue boxes. Often, I would repeat the same steps and insert the citation without any problems.

Much later, when I looked back through the "old" document (as opposed to just its bibliography) I noticed some of its in text citations had been messed up. They looked as though new citation field codes had been added into the existing field codes, even though my cursor had only been in the "new" document. It even had "citations" in the middle of the Table of Contents which I wouldn't even know how to do if I tried!

So I changed to having the old document on my laptop and the new one on my Mac, and the dialogue boxes stopped appearing. When I went back to having both documents open on my Mac again it recurred.

Now it's possible that I was accidentally moving my cursor into the old document during this process, but given how often it was happening, that seems unlikely. And I've had problems with Microsoft applications not playing nicely with applications when it's in Full Screen mode, so I'm wondering if a similar thing could be happening here?

Rhys: Do you think you've only seen this when using Split View? I would certainly believe that Split View could confuse Word in terms of which is the active document.