That's strange as it's working fine in IE for me here using the Dahua plugin 3.0.0.3 - the Dahua Plugin Class extension is definitely enabled within IE?
I think I've narrowed it down. When running WebPlugin3.3.27645.0, my SD49225T-HN works great, but both of my SD59225's won't work. If I upgrade to WebPlugin3.3.60413.0, then the 59's work, but the 49 doesn't. Having both installed at the same time works only with whichever one was last installed.
I've upgraded the firmware on both the 49 and 59 to the latest. A bit of a pickle.