Hmmmmm almost the same thing happened to me. The low pass output stopped working, though in my case the bandpass and high pass still worked. I contacted Robert and they sent me a new one, which is working fine atm. Maybe there is a wider issue here?
I'm collecting dead wasps here; and send living ones in return! For free! ;-) I remember an issue we had some time ago with the vactrols; causing non-working filters and sometime this happened intermittent. It's not unlikely you ran into the same issue. I will send a new one, of course. Please drop me a line by email, thanks!