I have been having the same problem and have contacted Agile and this is what they said

Hi

There was recent change in Yahoo Messenger which is why you are experiencing log in problems.

Our team of engineers are working round the clock to fix this problem.

Once that is done you can expect an update sent to your phone which we expect should fix this problem.

As this is unforeseen - we apologize for any inconvenience caused and assure you that we are scrambling ALL resources to get you back up chatting on Agile Messenger.

Thank You
- AgileMobile Helpdesk-

So who knows when it will be working again.

Is.