A Process Monitor log might reveal what's happening. (Please also let us know the path(s) involved so we can find the relevant info in the log.)
A screenshot of what's being double-clicked (or typed into the path field etc.) and the error message could also be enough to suggest possible causes.
Is Opus running under the same credentials and elevation levels as File Explorer? Running it elevated can mean it has different permissions on network shares, or sees different mapped drives, for example.