To check if it's related to the Opus configuration, I would do this:
- Backup your config.
- Uninstall Opus (this will wipe the config, so do the backup first).
- Reboot.
- Download and Install Opus.
See if double-clicking notepad.exe works now.
After doing the test, you can restore your config backup to get back to where you were. But don't restore it before testing, else it would defeat the purpose.