So it works in Opus with shorter paths?
And closing the viewer pane makes no difference?
Send us a Process Monitor log of the two files being double-clicked in both Opus and File Explorer, as well as the names/paths so we know what to look for, and we'll take a look.
Edit: It's also possible anti-spyware or similar software is blocking Opus, but not File Explorer, from opening/launching documents, which we've seen in some cases. This can include the built-in Windows Defender. Problems caused by Windows Defender (strange behavior when using Opus)