Yep, also having trouble with logging into (only) my yahoo account on my TP. Years have gone by without problem either on my TP and Pre2 but I've been experiencing log-in problems on the N920 since early June when Yahoo asked me to change my password due to some security account.
The problem first exhibited itself on my WP8 Nokia 920 (sadly, it replaced my Pre2) using the email app. Despite communications with both WP and Yahoo support, I am still experiencing frustrating log-in problems with my yahoo account on that device. Then yesterday, the log-in problem also occurred on my TP using the email app. Still can't log-in to yahoo from the TP. Interestingly, I can log-in via the browser on both the N920 and TP.
I've tried all the solutions recommended in various forums, have deleted and re-added the account several times and have reset the password at least three times at the behest of yahoo customer care. No joy.
If anyone has similar problems and has found a solution, please post.