If you are using the hosted version and cannot get the mobile apps to work it could be one of two things:
1. You've entered incorrect settings. Username, password or the helpdesk URL is incorrect. Please note that on the hosted version the URL should always end with "/helpdesk".
2. Networking issues. Your helpdesk site is inaccessible from the network you are currently in.
However, on the server version multiple problems can occur in the addition to the two above. We've described all of the known issues in the manual: "
Fixing common problems". Please go over it step by step. It should fix all your issues.