How to recreate an OPC UA application instance certificate

Likely, there is something wrong with the client application certificate created for the client application. Try this:

1. Download and install https://kb.opclabs.com/Tool_Downloads#UA_Configuration_Tool .
2. Run it.
3. Switch to the "Manage Certificates" tab.
4. Make sure that "Store Type" is set to Directory.
5. In the "Store Path", drop-down the list, and select the path ending with "Machine Default". The, press the "View Certificates" button. This is the store for the instance certificates of the UA apps on this computer. Delete all of them (right-click and choose "Delete"), or at least those that corresponds to the apps that have the problem.
6. Restart your application, making sure it is done under elevated privileges (so that it has write access to the stores). This should re-create the app certificate and store it where it belongs to - if that was the cause of the problem.