I'm experiencing this problem as well, on both my Android devices running Aqua Mail with Yahoo accounts (IMAP). It won't load any mails. However, as for my case, I believe that it's caused by some Yahoo server problems. Webmail is working fine. Other, non-Yahoo accoints are fine as well. On my desktop, Thunderbird only loads the first 200 (oldest) mails in every folder on my Yahoo account.
If anyone has more info on this issue, please share :-)