If Opus 11 is still on the system, please run FileTypeDiag and click save without typing an extension, to save a list of your folder-related registry settings. If you zip the result and post it here we can see if the problem is in the registry.
Please also make sure Opus is not running with UAC elevation, as that can break Explorer Replacement. (Opus has its own UAC support built-in, and the main Opus process should always be run normally, without elevation.)