Why UserChoice is still actively protected (despite UCPD being disabled)

Instead of the Windows 10 build-in OpenWith dialog I use a legacy implementation that uses the look of Windows 7 (please no comments on this one, I like it that way and triple verify only serious software is used/downloaded). This worked for years, I just have to set UCPD.sys startup to disabled for once. But now another process actively blocks the editing thing in the registry. I am 100% sure it's NOT ucpd.sys, its driver is deactivated as well as its self-healing task planer entry (sc query ucpd and sc qc ucpd tell me). The error phrase indicates it's NOT a permission but a handle issue (my permissions are sufficient and without any deny rules). There exists no defined policy that would cause that ("PolicyPlus" told me). Sample: All actions in this subkey are actively blocked by ??? (it applies for other filetypes as well): HKEY_CURRENT_USER\SOFTWARE\Microsoft\Windows\CurrentVersion\Explorer\FileExts\.jpg\UserChoice Conclusion: it's not about lacking permissions it's not about a policy it's not about ucpd.sys Does anyone have a clue which process (maybe another secret service) might actually responsible for that? It would help me a lot (others as well I suppose).