Create merge request fails with Proxy Timeout

When trying to create a merge request from a newly pushed branch, it always fails with HTTP 502 Proxy Error The proxy server received and invalid response from an upstream server. The proxy server could not handle the request GET //merge_requests/new Reason: Error reading from remote server.

There are no errors showing in the gitlab logs.

Using the "New Merge Request" button from the project itself works correctly.

The same error can be reproduced by pressing the "Compare branches" button without first selecting a sensible target branch.

Seeing this in 7.10.4 and it worked in 7.9.4 (do not know about anything in between)