Most likely cause of this is you were logged into your site on 2 machines or using a persistent cookie. You changed your username or email on one machine so now the cookie on the other machine is invalid and does not match your user but the cookies say you are logged in and have the correct edit roles, so you got a null reference on the user object when trying to save because no user matched your cookie that was created before you changed your username. Logging out and logging in again would fix your cookies and solve the problem.
I don't think it was a hosting issue at all.
Hope it helps,
Joe