Hello.

I would like to first say that UnsubscribeXXXX methods are designed to work asynchronously, same as SubscribeXXXX. That is, it would be normal to receive *some* notifications after the method returns. But I understand that this is not your situation - such notifications should stop - and not last 50 minutes or longer.

I cannot come up with a reason or a known cause for this behavior. I am not sure if by "non-reproduable" you mean that it has occurred just once, or that is occurs from time to time, but you cannot tell under which circumstances. If it happens from time to time, first thing I would suggest to do is to upgrade to recent QuickOPC version and see if that resolves the problem. If you are not license, I can give you evaluation license, free of charge. And if it resolves the problem, a significant discount on version upgrade.

Regards