I have consistently been having login problems with MyYahoo via AvantGo. It's also noted in the AvantGo support page as an issue. Almost caused me to dump AvantGo altogether.
But I was able to fix the channel finally by removing it and then adding it again. It works great now. Can't say it will work for everyone, but worth a try!
PS. I found that out by accident. Originally, I was going to replace the standard Yahoo channel with a custom channel based on the the thread for using iSiloX with the MyYahoo mobile site. (
https://www.mobileread.com/forums/sho...s=&threadid=34)
The links worked great from my PC, but when I tried to use the links in a custom channel it didn't work. Probably because of the cookie issue. In fact, some of the urls don't even contain all the necessary content in them, so it must be basing the page off cookie content. And thus all the notes about enabling cookies in that thread.