development server has disconnected

[WinterLLL]

I started the 68. section - Dev Authentication and was about to finish, but the problem was I couldn't tweet like him in the video. I began to look for mistakes and saw that my AUTHENTICATION_CLASSES were named AUTHENTICATED_CLASSES, so I changed the name in my settings but then all the tweets were gone and just the tweet section popped up. A long time I searched for other errors but I couldn't find them and I didn`t want to start allover again so I began to copy the scripts in my project. Of course I made sure that I didn't change important settings or other things. The problem after this was: nothing is rightnow on my react-website and if I look in my console there is the error message:
The development server has disconnected.
Refresh the page if necessary.
[HMR] Waiting for update signal from WDS...
I would be super thankful if somebody could help me.