I have also come across this problem today after pulling updating my Rasa Docker image (:latest).
I pulled a few old tags and tested to see which image the bug snuck in.
1.0.0 to 1.1.8 works, the bug appears from 1.2.0 up.
Here are the console read outs:
rasa/rasa:1.1.8
docker run --rm -p 5005:5005 -v $(pwd)/models:/app/models rasa/rasa:1.1.8 run --credentials creditials.yml --endpoints endpoints.yml --connector socketio --cors *
2019-08-08 10:46:57 INFO engineio.server - Server initialized for sanic.
2019-08-08 10:46:57 INFO root - Starting Rasa server on http://localhost:5005
2019-08-08 10:47:04 INFO engineio.server - c7e89e35fe1745449843515507bb4e25: Sending packet OPEN data {'sid': 'c7e89e35fe1745449843515507bb4e25', 'upgrades': ['websocket'], 'pingTimeout': 60000, 'pingInterval': 25000}
2019-08-08 10:47:04 INFO engineio.server - c7e89e35fe1745449843515507bb4e25: Sending packet MESSAGE data 0
2019-08-08 10:47:04 INFO engineio.server - c7e89e35fe1745449843515507bb4e25: Received request to upgrade to websocket
2019-08-08 10:47:04 INFO engineio.server - c7e89e35fe1745449843515507bb4e25: Upgrade to websocket successful
rasa/rasa:1.2.0
docker run --rm -p 5005:5005 -v $(pwd)/models:/app/models rasa/rasa:1.2.0 run --credentials creditials.yml --endpoints endpoints.yml --connector socketio --cors *
2019-08-08 10:52:29 INFO engineio.server - Server initialized for sanic.
2019-08-08 10:52:29 INFO root - Starting Rasa server on http://localhost:5005
2019-08-08 10:52:35 INFO engineio.server - http://localhost:5006 is not an accepted origin.
2019-08-08 10:52:42 INFO engineio.server - http://localhost:5006 is not an accepted origin.
2019-08-08 10:52:49 INFO engineio.server - http://localhost:5006 is not an accepted origin.