I am getting a failure trying to add a task via 2/tasks/add.php
For most of the values I tried there is not problem at all (so I am fairly sure I am not doing anything fundamentally wrong), but apparently some characters in the string I send manage to confuse the receiving code.
Here is is a sample string (goes after '/add.php?' )that results in this message
I am guessing that it is an encoding problem. I'll bet that something inside the note is not being escaped properly which is causing it to be invalid JSON. Try removing the note and retry that query and see if it works.