I'm stuck in an endless loop with iCloud – I can't access Icloud because my username and password haven't been verified but when I try to verify them I receive a message telling me that my email address (username) is already linked to another Apple ID.

iCloud – can't access account because apparently my email address (username) is linked to another account. How can this be? I've only changed my username at Apple's prompting and my email address is the same it has been for over 15 years. Not being able to access iCloud is preventing me from upgrading all kinds of things – not least my operating system.