Hi Adelino,
Here is my best guess of the problem.
You edit your site sometimes from home and sometimes from work. At work you changed your username or email address on your user account. When you got home you were still logged in as the old user name because you had a persistent cookie because you check the remember me box when you login. So when you go to edit it sees from your cookies that you are authenticated and you are in a role with edit permissions but when you save content it cannot find your site user because your cookie no longer matches an existing site user (because you changed it at work) so it gets a null user object and causes this error.
The solution is to click the logout link then login again, or else clear your browser cookies then login again. Then you will be able to edit again without errors.
Hope it helps,
Joe