A side-effect of some of their recent updates no doubt. I hear they get a lot of Mobile help calls, but provide no phone support for the issues (No phone support for Yahoo on phones... ironic?) Anyway, the help for this is help.yahoo.com, go to the mobiles section, click on one of the listed problems. On most of them, the "solution" page will ask, at the bottom "Is this enough information?" Click on no, fill out the feedback form, and send away.

PRocedure from here is as follows.
Techs send you an automated unhelpful letter.
You respond to it restating the problem and info
They prioritize it and send live, email, support.
DON'T CHANGE THE SUBJECT LINE! TEch support via the web is thread based.