Requiring constant sign in?

After initial sign-in, app requires constant sign-in/password entry. Error message says "There was a problem with your Apple ID and password" and option to choose either iForgot or Sign In. This is happening repeatedly while trying to use the app. I have reset password, but app continues to require repeated sign-in...to the extent that I can't access app Help (as troubleshooting says). Any suggestions to fix? Note: I just "reinstalled" app on iPHone 4 after having removed initial install when new.