Conversion from EHP8 not possible?

Dear All,

we are converting a ERP6 EHP8 to S4Hana on premise 1511.

In Maintenance Planer i can proceed fine and get a Stack xml file. See Screenshots.

The Problem is that SUM says there is nothing to update and finishes the Upgrade process.

For mee it seems there is nothing to update, because we installed the EHP8 in the last week. But it should be possible to start the conversion anyway.

Any ideas?

Best regards!
Jan