Error getting OpenID token from homeserver (400: Content not JSON)

[turt2live]

May-26-2019 19:09:00.871 +00:00 error [matrix] {"errcode":"M_NOT_JSON","error":"Content not JSON."}
May-26-2019 19:09:00.872 +00:00 error [ScalarStore] Error in request: invalid status code
Error: Error in request: invalid status code
    at Request.request [as _callback] (/home/node/matrix-dimension/build/app/matrix/helpers.js:213:28)
    at Request.self.callback (/home/node/matrix-dimension/node_modules/request/request.js:185:22)
    at Request.emit (events.js:180:13)
    at Request.<anonymous> (/home/node/matrix-dimension/node_modules/request/request.js:1161:10)
    at Request.emit (events.js:180:13)
    at IncomingMessage.<anonymous> (/home/node/matrix-dimension/node_modules/request/request.js:1083:12)
    at Object.onceWrapper (events.js:272:13)
    at IncomingMessage.emit (events.js:185:15)
    at endReadableNT (_stream_readable.js:1106:12)
    at process._tickCallback (internal/process/next_tick.js:178:19)

homeserver_1 - 2019-05-26 19:07:21,686 - synapse.access.http.8008 - 233 - INFO - POST-1141 - redacted - 8008 - Received request: POST /_matrix/client/r0/user/redacted/openid/request_token?access_token=<redacted>
homeserver_1 - 2019-05-26 19:07:21,687 - synapse.http.servlet - 212 - WARNING - POST-1141 - Unable to parse JSON: Expecting value: line 1 column 1 (char 0)
homeserver_1 - 2019-05-26 19:07:21,687 - synapse.http.server - 85 - INFO - POST-1141 - <XForwardedForRequest at 0x7f88771b8eb8 method='POST' uri='/_matrix/client/r0/user/redacted/openid/request_token?access_token=<redacted>' clientproto='HTTP/1.0' site=8008> SynapseError: 400 - Content not JSON.