Can't create MR, hangs in "Almost ready" with 404 in background

When creating a Merge Request, it hangs in the “almost ready” state:

In the dev console, I can see that a fetch request is failing with 404:

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I have already created and merged a number of MR’s in this repo and never had this issue.
Tried closing the MR and opening a new one; signing out and back in; no result.

I have this same issue. I think it is a symptom of a wider issue Gitlab are having right now:

Can confirm MR did merge eventually (10+ minutes)

Yes, mine has now gone through too.