This represents a timeout during a screen transition.
This can be caused by a host 3270 write which doesn't change anything on the screen, or by a host write to complete a screen which takes longer than the active timeout value.
The screen.timeout value defaults to 20 seconds (20000 milliseconds) and can be increased if in fact you have a very long pause from the mainframe.
It could be that the ignoreemptywrites setting is getting you, as in a "non-update" from the host, this can cause this. To override, look into using the TEREADYWHEN tag to control the screen transition where this occurs. You can see which transition this is by viewing the trace file in the DevCenter.