Avalanche 5.0 and higher
The mobile device server is showing its status as dormant. When the mobile device server is in a dormant status your mobile devices will be unable to connect to your Avalanche Server, giving an "Agent not found message".
You can check your mobile device status by right clicking over the mobile device server and going into Properties.
Solution / Workaround:
In order get the dServer running again a universal deployment / synchronization must be done. Simply go into your task scheduler and schedule a deployment or synchronization.
If you are running Avalanche Site Edition (SE) there is no deployment option. You can force a synchronization / deployment by making a change to a software package, or creating a new profile (this can be removed after the synchronization / deployment has taken place)