Right-clicking a folder and picking "Open in Explorer" works, but using the File menu opens Explorer in the Opus installer folder instead of in the intended selected directory. (Not sure, but I guess it is the one generated by FileType CONTEXTMENU.)
Directory Opus Pro 12.7.3 (Beta) Build 6657 x64
OS 10.0 (B:16299 P:2 T:1) SP 0.0
I could reproduce this a few days ago, but no longer can and it's working fine now.
Is it the same for you, or still a problem there? I'm wondering if a Windows Update coincidentally fixed whatever was going wrong there.
If it's still a problem for you, does it depend on the folder (or parent location) at all?
It's also possible that what fixed it was me turning Explorer Replacement off and later back on, which I did earlier to test something else. That would re-write some of the involved registry settings, which may be damaged on your machine.
On 12.7.5 beta, the items from FileType CONTEXTMENU=Directory\Background are being displayed even with a file or folder selected, unlike with previous stable versions.