Someone had problem with 'moodle_user_create_users': http://moodle.org/mod/forum/discuss.php?d=184276
Writing web service sample test client, I discovered that if the params was an object, the curl library that I used produced this error. I had to cast the object into an array to fix the issue of the library. It is probable that the same issue exist in Moodle core / Moodle client. More over the error message was not very clear.
1- try to find out if the error message could be more helpful
2- can we reproduce this error with the Moodle REST client ?