Suddenly can't login to yahoo email from iphone because it cycles with something called cprodmasx

“cprodmasx” is ATT’s authentication system.

OK...well, what I get when I try to log in from my iphone is the normal login screen come up, I complete it like normal, then the website address bar at the top just cycles back and forth between att and cprodmasx.....never goes any further.

PS:
I also access my yahoo webmail from my laptop at home and it continues to login and work fine.

For the past few days, I've found that once I've logged off from webmail from any browser, I cannot login again from that same browser; the address bar/tab cycles back and forth interminably between Logout Successful and cprodmasx... signin.  I'm down to Firefox and Edge on one computer and Internet Explorer [!] on another—I'm doomed should I slip up and log off.

Thanks for that...sounds like something weird going on with att/yahoo and this thing called cprodmasx...not just a problem of mine / my iphone.

Surprised my earlier google search didn't bring up anything about this...you'd think att/yahoo has to know about it by now...but getting any kind of simple straight-forward interaction / communication with their support is impossible.

Anybody know if this email address is still valid for Yahoo customer service?  ( I've used it in the past )

[email scrubbed]

Or if anyone has another known working support Email address for Yahoo Email?

Thanks

I am having the same problem with signing into my ATT email. I have to go in though myATT, sign in to my account, and then click on "check email." I cannot sign in to sub accounts.

I tried to do that, but I don't know what the URL is for "myATT".  When I tried to sign in to "currently..." and to "yahoo.com" with my ATT email address, I get the same maddening back-and-forth cycling between "Logout successful" and "cprodmasx...signin".

To get to myATT try:  

https://currently.att.yahoo.com/

then click on myATT on the right side of the horizontal menu.

I cannot log in to my email sub-accounts through myATT only my main account.

Thank you, thank you, thank you!!!  It works.  I'm not going to risk logging off, though, lest it subsequently fail.  As "wjcutb" observed a few posts above, it's effectively impossible to get in touch with AT&T (or Yahoo, I imagine) tech support staff—so this workaround is a godsend.  I owe you an ice cream sundae.

Final update...speaking only for myself, Yahoo's reliability got bad enough in the past that a year ago I set up a new gmail account and switched over all personal email to gmail...leaving only companies / businesses using the Yahoo account to minimize my new gmail account getting out & around for fear of starting a new wave of spam.

But this latest fiasco has crossed the "life's too short" line...not going to continue putting up with Yahoo, now AT&T / Yahoo and their total disregard for anything that resembles even borderline customer service and considerate notification of changes to end users.

I just completed a few hours of notifying / changing my email address with all the remaining businesses / companies that still had my Yahoo email address, then completely purged all folders and emails I'd accumulated at my Yahoo account.

It won't even be noticed or make a bit of difference to ATT / Yahoo of course, but they've permanently lost me as a customer.

Conversely, Gmail has been 1000% solid as a rock / zero spam for a year now and that's more than good enough for me.

Thanks to all who pitched in on this thread and good luck to you.