UPDATE: We believe we have resolved the issue. There is no need to send in a support ticket, as we are aware of which players have been affected by this issue.
There is currently a known issue with the Challenge Event which is causing some players to lose connection during Challenge Event battles.
We are currently investigating this issue here and will try to get it resolved as soon as we can.
When we have more information we will update this article.
Our apologies for the inconvenience caused by this issue!