Transfer-encoding: chunked

I have a REST API with JSON. One of the attributes of the API resource is a string where I have to supply a base64 encoded byte array converted to string (POST request).

Starting my REST API (dotnet core API) in debug with a breakpoint works, but if I hit the API without debugging, SOAPUI throws HTTP 500: Transfer-encoding: chunked.

Changing Chunking Threshold in Preferenced does not help. Regardless if empty, 0, -1, 999999999 it always gives the same problem.

I am using SOAP UI 5.5.0

Thank you for your help