Endless Signing-In Loop

When I signed in a few minutes ago, the www asked me to allocate a communities username.

So I entered the name I normally use and was told that it existed.

This is strange as it is a very unusual combination of letters etc.

The first question is: How can I find out if this communities username is me?

(Especially bearing in mind that the www will not let me sign in.)

If I invented a new username it accepted it (said that it was unused).

BUT then asked me to sign in again and said that the username had already been taken.

You get the picture - this could go on for years.

The final point to make is that I am religious at keeping track of my sign-in details.

Trying again while writing this text, it seemed to accept the original username, but then said : "unknown error has occurred" and took me to Manage your AppleID ... and in no part of this series of screens did it ask me for or show me pre-existing 'username'.

Thanks