Using
http://ptsv2.com, the only thing really different between what Insomnia (
https://insomnia.rest/) creates and what B4A creates is that B4A sets the Content-Encoding header (to UTF8). When I add that header via Insomnia, your site bombs out just like with B4A. So if you add
j.GetRequest.RemoveHeaders("Content-Encoding")
after you j.PostMultipart line, j.PostMultipart works.
Note 1: RemoveHeaders method relies on the OkHttp library
Note 2:
@Devv sent me his Insomnia configuration. There are two additional parameters that are required to do a successful post, but without these two, one still receives a JSON structure with the error message. With the Content-Encoding header set though, the site just bombs out, no matter what values are posted.