Sometime EthMan stops work properly - show all rigs offline and if i try close there are error message "Cannot terminate an externally created thread". Need kill and restart process via ProcessExplorer.
How many rigs in EthMan do you have?
Currently I'm rewriting some parts of EthMan, network routines too, probably it will help.
Confirm this bug from time to time. There were 25-50 nodes in the list.