classic html version
I still use the basic html version of Yahoo email from a couple of my PCs running Windows XP that can't run a browser new enough to run a newer version, and usually on my others too.
Don't know when this error began, been a few weeks since I checked my email, but when viewing an email the up/down arrows to go to previous/next email are grayed out, which might make sense for some reason for a few moments, but they stay grayed out and unusable. Pretty tough to check my backlog of unread emails since I have to go back to the inbox every time to read another message.
Would be very good for you to always offer a very basic html version like you do now for the reason I use it, for consistency for some users who simply have a hard time changing/learning how to do things, & a few other reasons I can think of, even if you upgrade your "normal" email interface programs. An option people could rely on and fall back to if for ANY reason they have personal or technical reasons they don't want to or can't use the normal interface.