Really depends. It’s probably not a hacking attempt if it comes from the phone’s mail app, but if you have a special Yahoo branded email app, then it might be. Most apps keep a token rather than having to sign in each time, but sometimes those expire, or change formats, or a number of other things. It could even just be an old password, or even an out of date app. Best bet would be to note which application is giving the error and then make sure of your settings in there. Sometimes it takes removing the account and then setting it up again, but not usually. Also just make sure it’s updated. If you’d like, grab a screenshot and put it somewhere we can view, happy to take a look and track down some steps for your phone OS and Yahoo. No worries either way, just realize the above might not be as helpful as intended ;)