Openhab 3 Rest API problems

Hi everyone,

I have been using openhab for about 2 years now, and i must say it is a great Software and that i have bearly scratched the surface of knowledge.
That would explain my Problem at the Moment. I have updated openhab 2.5 to 3. Repaired some of the issues, with timecalculating rules, Bindings, Services and things. What i haven’t figured out is how to get the rest api to show/work.
Everytime i use the openhab ui and go to the developers tools an klick on api Explorer i get this Massage.

It feels like i have looked through the entire Internet and haven’t come to a result.

Can i restart just the api part, is there something messing in my config. I have absoluty no Club.

I hope you guys can help me.