I do not see a problem with how you described your code.

I have created a C++ program her that does comparable thing, and it works well - i. e. I could not reproduce the problem. This is with our "kit" OPC Server. I am actually calling the AcknowledgeCondition from the event handler, which is on other thread as well.

Questions:

1. Have you tried to call some other method, besides acknowledgement (something harmless - e.g. querying for categories)? Does it work, or does it also give the same error?
2. Can you try your code against our server? If so, how does it behave?
3. Are you calling CoInitializeEx with proper arguments (most likely, for "free threading") on beginning of each thread that you use?

Note:

The AcknowledgeCondition method is quite sensitive, besides other things, to the proper value of the ActiveTime parameter - it must match the time earlier obtained from the corresponding event notification. If improper ActiveTime was the problem, however, an expected error code would be E_INVALIDARG.

Would it also be possible that you send me (parts of) your code - for quick check whether I do not quickly "spot" some obvious problem?