I'm having the same issue on a Mac with the Acrobat Reader DC. Only the app program. Everything fine via web browser or mobile. Did a full uninstall and reinstall. When "sign in" pressed, it opens the window, but hovering mouse now gives greay circle and slash. Can hit "tab" which permits email address entry. But "Continue" buttin not possible, and hitting Enter/Return does nothing. This was working fine until recently.