Listing user's courses: errorcoursecontextnotvalid

Hi,

I'm currently testing differet webservice functions for a remote access to our Moodle. One thing I'm trying to is to list (1) all courses available and (2) all courses available to a certain user.

I created a webservice with all the proper functions, rights are assigned, too (e.g. moodle/course:view). Nonetheless, when I try to do the following:

https://myserver.org/moodle/webservice/rest/server.php?wstoken=7xxxx&wsfunction=core_course_get_courses

I get this error message: errorcoursecontextnotvalid

The only thing that helps is to use the webservice user as admin. But I don't think this is the proper solution to the problem. Does anyone have a suggestion or hint?

Regards