by Anne Krijger.
Hi Maciej,
It's always useful to know what version of Moodle we're talking about.
My suggestion would be to find out where in the code this exception is thrown (that should be easy to find) and possibly use a debugger to see what exactly is going wrong.
I only found 4 places in Moodle 2.4.x where this can occur.
If you feel real adventurous you could write a Unit test for the webservice and use that while debugging.
Anne.