One thing that jumps to mind is that there is an ASCII code for an EOF character. While highly unlikely, it is possible that that some text field in a table has had a character inserted into it that is being interpreted as the EOF char. I’ve had issues where users copy and pasted text text from web pages or Excel, and the text ended up with control codes (characters with ASCII <32) The RS (Record Separator) character is common.
Since it happens when the client program is sitting idle, I’d check all the tables related to the info conveyed back and forth by the system monitor. Maybe even try exiting the system monitor, and leaving the client open to see if it appears.