the invalid return value is usually due to some configuration problem or maybe a bug in a WS. The weird thing is that the courses disappear afterwards :/
Can you please send us a test account to email@example.com so we can check it? We need the Moodle URL, username and password. Also, please indicate the name of a course causing this problem so we can search it in the app
To be able to determine what is causing the "invalid return value" error we'll need debug mode activated (configured by admin). If this is a production site you should enable it only for the test. I'll tell you how and when to enable it when you've supplied us the user account.
thanks a lot for your assistance and help.
I've just sent an email to firstname.lastname@example.org with the data.