In cases where there are more than one CIP device, (HMI, PLC, etc...) they share a single data stream in the controller. We keep track of how many devices connect to the data stream and when they all disconnect we close the data stream.
There is currently an issue in OS versions up through 2.41 which can cause the data stream to be closed and CIP communications stops even though there are devices still using it. The count of devices is incorrectly decremented by two when a device goes offline which results in a zero count when devices are still attached. If you are using just one CIP device, this issue will never be a problem. If you have multiple devices and just one of them goes offline several times, all devices will loose their connection.
To recover communications, ether the controller must be power cycled so that it re-initializes the CIP device count, or all of the CIP devices must be shut down then all restarted which will allow the controller to detect the correct number of devices.
The issue has been identified and will be fixed in a future release.