On three separate occasions I've had my 875's remote stop working out of the blue. I have a harmony remote as well, and each time, neither remote would work. The first time it happened I took it to a service center who couldn't find anything wrong, but updated the firmware and replaced the remote anyway. It worked when I got it back
The second time, It started working again a day later.
The third time was today. I spent two hours trying to troubleshoot, and I finally figure out the problem:
My roomba base is in the opposite corner at ground level, and has a direct line of sight to my 875's remote sensor. Today my roomba happened to be stuck on the opposite side of the house with a dead battery. Under these conditions, I'm guessing the base station sends out some IR or RF waves to try to lure the Roomba home to charge, and those waves are interfering with the 875's IR system.
When I blocked the line of sight from the Roomba's base, the remote started working again! Also, when I return the Roomba to it's base, the remote works fine. But as soon as the Roomba is dead and not at home, the remote stops working. I repeated this at least 7 times to make sure, and each time i got the same results.
Yeah, this is probably very random, but if anyone else is having remote issues, look around for any possible sources of IR/RF interference.
I'm just glad I don't have to tear my system down to take my out of warranty 875 in again.