apt update: repository regolith-linux does not have a Release file

Good morning, I am running Raspberry Pi OS (Debian 11 bullseye) on my Raspberry Pi 400. I have the window manager i3wm installed. Here's my problem: As soon as I try to run

apt update

the following error message is shown:

: The repository 'http://ppa.launchpad.net/regolith-linux/release/ubuntu  mantic Release' does not have a Release file.

because of that I can't update my system and/or install anything. How can I fix this? Or is there a way to "exclude" the regolith-linux repository from apt update ? Thanks, Martin