Firstly which box do you have? The V6 (with 6 tuners) or the TiVo (with 3)
How many tuners are in use when you get this error? And are you saying "some channels" will not load, or "on Demand is unavailable"? Most VOD will use one of the tuners, but it might help diagnosis, knowing what you're doing to trigger this error.
Fair enough. Well on the front right of the TiVo box there's 3 red lights that illuminate when each of the 3 tuners is in use. Set a couple of recordings for programmes that are on "now" if you want to see what I mean.
In terms of the message, so the box is saying "VOD is unavailable" when just selecting a normal channel.. well apart from asking whether any particular channels are affected, I'd be checking whether the box has been rebooted recently?
Thanks for posting and welcome to the community. I am sorry for the On Demand issues. I have ran a background check on the account and can see there is a congestion issue affecting the service. F007481449 is the fault number. The estimated fix date is the 29th January 2020.
I'll PM you now to arrange the credit you're owed for this.