During upgrade the following error message is shown:
[LOGTEE]: Some of ppm mirrors could not be accessed. Please check if machine with installed mirror is running and accessible from Management Node host. As a last resort, you can delete mirror using 'ppm_ctl mirror remove' command. List of problem mirrors: [LOGTEE]: [LOGTEE]: Mirror ID URL [LOGTEE]: 1 http://188.8.131.52/tarballs [LOGTEE]: Error: HTTP Error 500: Internal Server Error
Only one repository should exist in PPA. It is located on PPA Management Node. If any other repositories are registered, it should be deleted.
PPA Management Node > Infrastructure > Package Repository > switch to 'Mirrors' tab > only mirror which points to PPA MN should exist.
PPM repository works under Apache web server. It should be available via HTTP, but not HTTPS. Make sure that there are no any redirection from HTTP to HTTPS in Apache configuration.