It used to be just when EPL games were on BT Sport (usually once a week).
It's now that, plus every Champions League and Europa League matchday too, now that BT Sport have exclusive rights.
We've all made sure that VM are fully aware of the problem. They haven't fixed it in 20 months. In that 20 months, Virgin Media have continued to use TV Anywhere to close sales, and as a means of justifying yearly price-hikes, so more & more people are trying to use what was an already failing service. It can't cope.