Hi Rigoberto, i dont exactly remember whole process, i was having multiple problems (your error sound similar) and in the end i dropped this idea of feature for now. I think moodle ws are great, but i am missing some features and i hope in future moodle will get this work better.
If ill have some time, i will do testing for your error, but i dont promise that (busy times).
For now we are thinking about making some .php code with moodle functions to maybe create own web service, or plugin with this - we will see.
And for personal info - im using Postman/ARC and im java dev... our company using moodle to some courses - mainly scorms and light administration. Whole logic is created in java - email notifications, display courses, additional administrations of users, ui for HR people and more - our internal system for employees and management. And here we want to have something like that, that people can save assignments from external app, as is ours.
by Jozef K.