"Could not decrypt key" upon login

[PancakeConnaisseur]

Regarding the IBootstrap warning: It is not the cause:

App.php is deprecated

This is a warning directed at the developer of the notes app. Unless you are willing to participate in the development, there is nothing you can do about it. But this is also merely a warning, it does not affect your Nextcloud instance in any way. And as kesselb said, you should not have log level at 0, because you are getting a lot of information that does not concern you as an administrator.

The other message seems to be the right one. You have some kind of problem with decryption. Do you use end-to-end encryption?