Be aware this is the english forum, so posts need to be in english
you can only use one steam account linked to one swtor account , that error is seen when another swtor account is linked to that steam account
According to everything I found it leads to the same thing you say but I don't know what is linked, I don't have how to know which is the possible "linked" account and the one that was listed as the one I unlinked immediately but it didn't work anyway.
the linked account will have steam linked at https://account.swtor.com/user/steam
you can try input all emails into https://account.swtor.com/user/displayname/recover and see if it provides another swtor account
