Platform? (Steam/Windows/PS4/PS5/Mac etc.)

Windows.

Does this only happen to one character or all characters?

Only the one character that is in an Endwalker area (as far as I know).

Do you have the Endwalker expansion installed/registered to the account for the corresponding platform or only Early Access? Note that since Early Access has ended as of 12/10/2021 that you will need to register the actual registration code to continue playing content associated with it otherwise you will lose access to the expansion content.

I only have the early access linked to my account - not the actual expansion itself. I was in an Endwalker area when I entered the Duty Finder to do a raid - Void Ark - which is a Heavensward area. Without the actual expansion linked to my account, is there no way for me to get access to my character and continue to play it in non-expansion areas or am I stuck until I actually have the expansion linked to my account?