Now, on my Zotero, there is a new item tag, #broken attachments. But when I go to these attachments, the links are, in almost every case, not broken, but intact and working. It has flagged almost a thousand of these in our bibliography project. Something is not working right here.

Emiliano's storage scanner adds that tag.

Odd. I didn't add any new add-ons--it just suddenly appeared at the top of the tag list in the bottom of my group libraries window on the left. It also shows up at the top of the tag list on the page for my group on the web:

https://www.zotero.org/groups/73933/h-buddhism_bibliography_project/items/top?

I checked my add-ons list, and there is nothing there.

This doesn't do any real harm, but it is useless and misleading, since the so-called "broken tags" are actually not broken.

If you don't have the scanner installed, then it obviously isn't the scanner that is generating these tags. I don't know how they got there, even if, as @adamsmith points out, the scanner is the most likely culprit to have generated them.

The only thing I can think of (still assuming that you do not have the scanner installed) is that you have this library shared with someone but haven't synced the attachments, and that this other person has run the scanner. That would tag the attachments as broken on their end (and they would be), but the tags would sync to you.

If you want to fix this quickly, you can just delete the #broken tag. No need to install any plugins.

I don't trace who runs the scanner, so you'll have to find out who did and discourage them from doing so in the future yourself. The scanner does not distinguish between groups and personal libraries, it just runs over all references.