Hi @alvarolb and thanks for your quick response 
Requestcatcher was indeed a useful service, and it did reveal something.
Here is the output from postman, which obviously work (but i put in the body/json data for reference):
POST /test HTTP/1.1
Host: ct.requestcatcher.com
Accept: /
Accept-Encoding: gzip, deflate, br
Connection: keep-alive
Content-Length: 45
Content-Type: application/json
Postman-Token: 5bc407cf-d81b-4ee4-ac88-225f498df734
User-Agent: PostmanRuntime/7.26.5{“latitude”: 39.416775,“longitude”: 89.70379}
Now, here is the output from my Arduino code:
POST / HTTP/1.1
Host: ct.requestcatcher.com
Accept: /
Connection: Keep-Alive
Content-Length: 40
User-Agent: SIMCOM_MODULE{“longitude”:40.0015,“latitude”:90.5032}
Indeed, the “content-type” is actually missing, so that probably explains why the request is rejected.
I don’t see why this line of code in the Arduino project should not work anymore though. As far as I can see, the content-type is set correctly:
SIM900.println( F(“AT+HTTPPARA="CONTENT","application/json"”) );
Anyway, since this might not be a Thinger related issue then, I might have to bring this to another forum… unless you have any ideas of course