What does the error message say exactly? Please post a screenshot.
If you go into the target of the symlink in Explorer and double-click the file there, do you see the same problem?
Opus may be resolving the symlink to its target before asking Excel to open it, but it should not make a difference.
If you're using Windows Defender, or any similar anti-ransomware protection, that can get in the way of programs opening and writing to your document files/folders. It would normally be Excel writing to the file, not Opus, but it could be treated differently if it's launched from Opus (or other software) vs Explorer. See Problems caused by Windows Defender (strange behavior when using Opus) if that seems like a possibility.