I have the same problem with authentication in Moodle Mobile (for iOS) with my site, for Android everything works well. I have already tried everything without success.
I've followed all the instructions detailed here : https://docs.moodle.org/27/en/Moodle_Mobile_additional_features#Installation
But the problem persists. The Debug ADOdb option is also disabled. BUT the problem continues to exist.
please can you tell me what could be the problem in my case ?