I encountered a similar problem a few months ago. I have two separate networked TiVos. One behaved as you described, the other was OK. Solution: I "rebooted" the offending TiVo by pulling its power plug for a couple of minutes, and then reconnecting it. Problem solved. Apparently, some part of the resident software in the offending TiVo unit had become corrupted.
I applied the adage from the Windows world: "When in doubt, reboot."
Hope this solves your problem.