I'm having the same problem. I have four email accounts, and only Yahoo is exhibiting this issue. I've found I can temporarily fix this by re-entering my password in mail setup on my iPad, but if my iPad sleeps, the issue returns. I've has the same problem on my iPhone, as well, which leads me to believe its the new iOS upgrade.
Rumor is there's a new iOS a few weeks away..but this is a disappointment.