Refresh Token - Empty reply from server

We recently started having problems with our refresh token call. We are getting this response (after about 20-30 seconds):
curl: (52) Empty reply from server

Any ideas on what might be going on? Other calls into Hydra work fine. The refresh tokens do get invalidated, so something is happening within Hydra when we make the call. But we never get the response containing the new tokens.

Please open an GitHub issue and include essential information, like log files and software version.