I got to the Same issue,than everybody else Here. So after checking www and searching myself.for a solution with npm and nextcloud, i found a pretty easy solution.
First i thought, If i hide the Upgrade via advance Config in npm it should Work. But Here comes the Twist, the header was redirected to my nextcloud Webserver (apache2) which tried to answer.
So after a few Tests, i also Neef to ad unset header for Upgrade.
After an restart of my nextcloud Webserver → tada it works! A friend of my ist now able to Access the Web Page and also nextcloud App ist now working again.