Hello Koen,

Please let us know if you are getting this issue when devices are online or offline.This error message generally appears when MobiControl is in the process of sending actions like script, file sync, package etc. down to the device and the device is not connected. Every device when connected to the server holds a socket connection in order to communicate.At certain intervals the server pings the device to check if they are still online using that socket. Also, the server will use that socket to send any action to the device.
If for any reason the server initializes a communication to send an action and notices the device is offline it will log that error and close the socket that was assigned to that device. The way to "fix" the problem is to bring the device back online, this will happen as soon as the device gets connectivity and the agent automatically re-connects.