Moodle for mobile

Hello forum participants, maybe someone here could point me to a spot where to dig deeper in order to solve the following problem: our setting here : moodle v. 3.1.1+ (on debian, default mobile plugin may 2015, no mobile additional features plugin (so far)) moodle mobile app 3.1.2 (on iOS) since the update from 2.8.7 to 3.1.1+ students are no longer able to get their enroled courses on the front page of the app. although, if I enrol a student freshly in a course (manual enrolment, via web interface), the respective student will see this course in the app. another strange thing is, if this student then searches another course (for self-enrolment) on the app, and keys in the enrolment key, an error pops up (- in english s.th. like: found invalid return value-), and the former course (that we enrolled him/her manually) already disappears from his frontpage in the app. when double-checking in the web frontend, though, the user is self-enroled in the course. I've already studied the admin documentation for moodle mobile very thoroughly, but couldn't find any mismatch so far with our config. is it possible to dig deeper into this specific problem via s.th. like this: https://moodle.org/mod/forum/discuss.php?d=328894 ... and if so: how? other hints I've found so far: https://moodle.org/mod/forum/discuss.php?d=329034 (we've tested this, .. without success) https://moodle.org/mod/forum/discuss.php?d=337532 (but we are not quite sure whether this applies to our problem) any help very much appreciated, as in our former moodle installation v. 2.8.7 all worked smoothly Thanks a lot in advance, Bernie