Hello,
this with an old version of QuickOPC which we no longer officially support. But I will try.

It can be related to Windows file access permission to the folders that hold OPC UA certificates. Those are the various folders usually under "C:\ProgramData\OPC Foundation\CertificateStores". Normally, they need to be secured secured in such a way that the user account your app runs under has read/write permissions to it (but at the same, keep the bad guys out). It is possible that some permission is missing. Subfolders, unless they inherit the permissions, need to be checked too; from the error, specifically, the "C:\ProgramData\OPC Foundation\CertificateStores\MachineDefault\certs" and "C:\ProgramData\OPC Foundation\CertificateStores\MachineDefault\private" .

Also, while you are at it, back-up the contents of these folders, remove all files from them, and rerun the application. In some cases it may help.

Regards