Error retrieving metadata for SLE-Module-Legacy12-Pool: Not ready to read within timeout

Hi all,
I want to update the operating system from SLES 12 SP5 to SLES15 SP1, but I get following issue during the run migration:

Executing ‘zypper --root /system-root --non-interactive --gpg-auto-import-keys refresh’

Error retrieving metadata for ‘SLE-Module-Adv-Systems-Management12-Pool’:
Not ready to read within timeout.

Could somebody help, please?

Thanks and regards,
Mimi