For Android Opera Mini users experiencing problems accessing Yahoo Mail:
The Opera Mini team is apparently aware of the problem and working on a solution (see: https://forums.opera.com/post/88968)
A temporary fix is to:
a. Upgrade to / install the 'new Opera Mini' from the Play Store (version 12.0.1987.xxxxx)
b. change 'Extreme' savings mode to 'High' savings mode, by clicking on the red "O" Opera icon, and clicking on the 'savings mode' next to the graph indicating your data savings from Opera.
c. If that doesn't work, and if you still have access to the setting for 'single column mode', remove the checkmark from that.
This worked for me. I hope it helps any of you with the same problem.