LimeSurvey: wrong decryption key (AARGH)

If encryption shit fails in your LimeSurvey, try this:

TL;DR: Save again your SMTP password.

Have you fixed? Great! Close this browser tab.

Is the bug already there? Oh no! Probably you are an idiot and you deleted your encryption keys.

You have a backup. Isn’t it? Uh?