The importer throws the error "Exception: No site found in root line of page xy" when running - why is that?
The importer accesses your TypoScript settings during execution. To find this, he climbs up the root line from the data container and tries to find a TypoScript template. If it doesn't find a TypoScript template or root page, this error occurs.
But the error is quickly corrected. Just move the data container for the OpenImmo records to a side branch. If you have several side branches on which real estate is displayed, it doesn't matter which branch you move the data to - it is only important to know that the importer finds its configuration in the TypoScript template.