On one floor of my house, one cable drop feeds a Samsung cable box that feeds a TV set. In another room, another cable drop feeds the MoCA box which connects to an Orbi satellite router. That all works just fine.
On another floor, the only cable drop is the one that feeds another Samsung cable box that feeds a TV, so I put in a MoCA-compatible splitter. One branch goes to the MoCA box and then to another Orbi satellite and the other branch goes directly to the TV. The network worked fine and dandy, but the cable box would only go into "Err"state.
So I got another POE filter and put it inline with the cable box and now everything works. I'm still not sure why only one cable box was affected -- they're both the same model. But if you run into cable TV problems, this might be an easy fix.