I don't use Yahoo mail, and you didn't share a picture of what you're seeing. Going by some other badly designed apps, I wonder if there's a pullable tab/pane that you're not noticing that would bring the view out (like a sliding drawer).
Edit: I went to install it, to try and help. There's a notice that they're aware of issues when trying to view mail. They suggest clearing your cache for the app should help resolve the problem, which OP has already done. :/
Edit: I installed the app. Awesome! The message pane flashes every time you pick a new mail to view, and the loading spinner just spins. Oy.
Tips for a pleasant XDA:
Search before asking. Someone has likely already asked.
Spell your words completely, and use punctuation. It confuses people when you use "u" and "ur" and "dat", or when you wrongly use "there/their/they're/your/you're".
Be nice. Don't be rude, just move on to another post without leaving a comment. We are not 7 years old.
And PLEASE, do not ask for the "best" anything. We all have our preferences.