iTunes producer refuses to recognize my Apple ID
I have uploaded several books to iTunes with iTunes producer. After upgrading to El Capitan (have no idea if this relates) iTunes Producer claims "Your Apple ID or password was entered incorrectly." I rentered both several times, then clicked through to change the password. iTunes Producer still gives the same error message with the new password.
How do I straighten this out?
John
Mac mini, OS X El Capitan (10.11.3), iTunes Producer