Unable to update ispmanager panel on CentOS 7

Symptoms

Panel installed on a server with CentOS 7 can not be updated.

When making update attempts, an error is registered in the log at /usr/local/mgr5/var/pkg.log:

Could not retrieve mirrorlist http://mirrorlist.centos.org/?release=7&arch=x86_64&repo=os&infra=stock error was
14: curl#6 - "Could not resolve host: mirrorlist.centos.org; Unknown error"

Causes

CentOS7 support has been discontinued by developers, its system packages have been moved to the archive repository.

Solution

Replace the links in the repository files with the correct ones by running the following two commands:

sed -i 's/mirrorlist/#mirrorlist/g' /etc/yum.repos.d/CentOS-*
sed -i 's|#baseurl=http://mirror.centos.org|baseurl=https://vault.centos.org|g' /etc/yum.repos.d/CentOS-*