Exception Caught in Thread ControllerConnectionThread

I have a CB3 series UR10 robot. I have upgraded it from version 3.11 to 3.15.4 going through versions 3.12, 3.13 and 3.14. I have booted the controller on each upgrade and moved the robot with a program. I am now experiencing a problem when starting the robot. Sometimes (50% of the time or so) the controller will not start. I get a “No controller” message and the following exception: “Exception Caught in Thread ControllerConnectionThread”. Should I install a full version 3.15 image?