I am currently testing an External Service (REST with JSON response) on our test system and I am experiencing problems will all user related API calls (core_user_*):
core_user_get_users / core_user_create_users: webservice_access_exception
core_user_view_user_list with courseid 0: I am only getting status:true, warnings:, but not a single user listed.
I switched debug mode to "DEVELOPER", checked what debuginfo suggested and could not find any problems.
Because this is a test system I am using a user for this web service that has the Manager role and all capabilities checked.
Toke authentication works because I am getting authentication errors when changing the token.
Requests to other API functions like core_course_get_courses or siteinfo work fine.
Moodle version is 3.5.3+, build 20181204