How long has this been happening? If just a few hours you can probably just wait it out. WHen yahoo plays with their servers they just play with their servers and users never know and the problem resolves itself.
Beyond that if it’s been a while and the uninstall/reinstall won‘t work you can use Safari and the webmail interface to get by.
Yahoo is buggy. And I’m sure it has absolutely nothing to do with them wanting users to use their app. Which is another option for you, download and use the yahoo app and see if that solves your issue.