1- Enterprise Mobile Connector for JIRA add-on is not installed.
Solved! We are now allowing all users to log in even add-on is not installed. However, users will have limited access for features. (ex: can't create issues)
2- Wrong protocol selected (HTTP/HTTPs).
Solved! users are obliged to write the full URL.
3- JIRA is hosted in a local network and user is connected to the internet.
Support agent should ask users to connect to same network that is hosted on.
4- JIRA is hosted in local network and users are connected to the local network they are unable to connect.
Proxy is not configured on his phone, two solutions:
a- Ask him to configure proxy on his phone
b- Ask him to try to connect through IP address not URL (might work)
5- All above are resolved but users are unable to access JIRA
Probably his JIRA is down! check if he can access it from browser.