Assigning an issue to project silently fails, can't assign

We’ve been using the Projects beta for a while now, but in the last few days we’re no longer able to assign issues to projects. When I try to manually assign a single issue, the server responds with a 200 on the POST request but nothing happens, and the issue does not actually get assigned to the project:

If I try to do a bulk assignment, the server returns a 500, here’s the JSON returned:

  "id": "7fae63d9-1d15-4f65-ba47-e36a2292065b",
  "state": "error",
  "ttl": 604800

Any idea what’s up here?

I’ve actually come to learn that this was due to the 1200 issue per project limit. The UX is really bad on the issue queue side, but you get a clear message when trying to add it on the project page via the search: