Can't sign back in to Runescape without name change
Solution 1:
Runescape being a very old game, I've received many emails over the years saying that my account would be flagged as 'Inactive' and in that way my character name would be freed.
I guess maybe that applied to you?
- Your account is flagged as inactive
- Your username is taken by somebody else
- You log back in that account.
Now I don't have any knowledge of how accounts in RS are programmed, but I would guess that when you logged in your account everything was fine, but after a little while some checkup in their backend just realised there were now 2 active accounts with the same character name and this cannot stand. So it prioritized the one that had the 'fresher' character name, and thats why it kicked you back to the login screen asking to rename your character.
From this point, we can take all the guesses in the world, but only one person can actually fix this. RS Support. So contact them, and try to figure out the situation. At best you will just have to rename your character and everything will be fine from there.