I am not authorised to change the site appearance

Hello, when I try to create my first newsletter, the editor tries to open in the customizer, resulting in a message which translates more or less in "Ah, you're trying to cheat! You're not authorised to change this site's appearance!"

(I don't remember the exact phrasing in english for this error, but I believe you know what I'm talking about.)

Any suggestion?
(I already deactivated one of my plugins, which allows post editing in the customizer, but nothing changed.)