I'm having this problem where I'm creating a document set, and the New Document button is greyed out. I've searched the internet for answers, and I've tried them all:
Activating Client Integration -> was already activated, deactivating and reactivating didn't solve the problem.
Enabling content type management -> was already enabled, disabling and reenabling didn't solve.
Disabling UAC -> Major WTF of a solution. Tried anyway, didn't work.
Adding and removing different content types to the library. Didn't work. I even set it to accept all content types. Still didn't work.
I've also checked for any code that might be forcing the button to be disabled. There is none.
BTW I'm an admin and my permissions are set for full control.