Vaadin - SyntaxError:JSON parse unexpected keyword at line ...

Hi there,

there are similar issues to this one, but it has been more than 4 years since they were posted.
I built a small vaadin application with a running
server. I followed exactly every step which were demonstrated in an official video.

But there is one problem I have. After sending a request e. g.
I don’t get a proper reponse in my network analyize tool. There aren’t any problems directly in the
- evertyhing is displayed without problems. But what can I do to remove this message or to fix this problem:

SyntaxError: JSON.parse: unexpected character at line 1 column 1 of the JSON data Whats the problem?
Best regards,
Nazar Medeiros