When upgrading with the assistant from the stable channel all steps go OK but with the last one “Go back to your nextcloud instance to finish the update” the following error appears : Internal server error and a code : RkA5YPd1eE3SQ8EqznBJ. The server remains unaccesible.
If your web server is chrooted, try to create /etc/ssl/openssl.cnf in your chroot. I don’t understand why there is any dependency on this file, but you will get the “internal server error” if nextcloud cannot access that file.