Hello izzy 
that seems to work so far. I can register new users through this route. The users are added to the database and I can see them. The only problem remaining is, that I dont get an accessCode in the response after registering or logging in. Instead I just get a new error message saying: " We’re sorry but Speckle doesn’t work properly without JavaScript enabled. Please enable it to continue." Here is a screenshot of postman.
By the way, your github links are really usefull giving me more insight into how speckle works! 