I never have this problem with individual account. Only encountered this problem when move my accounts to a FA account. I have encountered this error on both the demo and live account. Tested with emini futures and stock. Stop price was hit, but order sitting on TWS. I am connect to the HK server.
If you click on "Order" there is an option (allow orders to be executed outside of regular trading hours). Did you check it?
If you want to rest the stop on your pc instead of with the broker, you can use this tool. http://mentalstops.blogspot.com/