Upgrade fails repoindex.xml not found

We try to upgrade SLES 12 SP5 for SAP Application to SLES15 SP5 for SAP applications.
We mounted the iso, booted and chose upgrade.
After selecting the partions, it started to refresh repositories.
We receive an error:
When hitting OK, we are back at partihtion selection.
Now we get a new error that product SUSE Linux Enterprise Software Development Kit has not been activated on the system.

But before upgrading, this product has beend activated on the system.
When rebooting to the original system, product is deactivated again.
What are we doing wrong?

Given the scc.suse.com URLs in your first screenshot, does the server have Internet access during the upgrade? Or are you expecting to upgrade solely from the ISO? If the latter you need to add “media_upgrade=1” to the kernel command line when booting from the ISO.