We have also tried the newer toolkit QuickOPC 2016.1 and result is the same.

Hello.

We have used QuickOPC 5.30 kit to develop a client application which subscribes to the Kepware OPC DA server tags and based on some numeric type tag values writes corresponded messages to the same OPC server string type tags.

Application worked pretty good until we increased number of instances up to 5. At this point application became frozen. Console messages indicate an internal error:

Exception Writing OPC Tag:
OpcLabs.EasyOpc.OpcException: An OPC operation failure with error code -1073430512 (0xC004C010) occurred, originating from 'OpcLabs.EasyOpcRaw.DataAccess.RawEasyDAClient'. The inner exception contains details about the problem. ---> System.Runtime.InteropServices.COMException: Write not completed. This error indicates that it could not be verified that the requested write operation was completed during the timeout period. It is possible that the write operation will actually succeed or fail, but later. Increase the timeout period if you want to obtain positive or negative indication of the operation outcome. Other reason for this error may be that under heavy loads, topic request or response queue is overflowing. Check the event log for queue overflow errors (if event logging is supported by the product and enabled).
--- End of inner exception stack trace ---

We have consulted with Kepware Tech support about this issue. After testing our application in their lab, Kepware response is:
The heart of the problem here is that the client is blocking on the OnWriteComplete call when it should not be doing so. The OPC DA specification does not specify whether the server side OnWriteComplete should be handled asynchronously or synchronously, so it is up to the client to handle both cases.

We have used EasyDAClient class in our implementation.

Do you have any idea what could happen and how to fix this issue?

Thank you in advance.

Mikhail