Please remove/replace current ones based on http://:
As problem seems to be related to the HTTP headers, therefore please replace above address with:
https://ftp.debian.org/debian/ bullseye non-free
or (only if preferred) some local Debian repository (with https instead http
deb https://ftp-stud.hs-esslingen.de/debian/ bullseye non-free
Please add this one in addition (optional for testing purpose but you might keep it within this .list file as well):
deb https://security.debian.org/debian-security bullseye-security non-free
Also NOTE that PureOS repository already starts with https (therefore shown incompatibility).