thank you so much for your advice!
Yes, we have the latest release of ckanext-showcase, namely the version 1.4.4. We use this Version together with CKAN 2.9.4.
We know that it fails on the showcases because rebuilding the index on regular packages (datasets) is working and stops as soon as the process reaches a showcase. If we delete the affected showcase and re-run rebuilding the index, it stops on the next showcase.
We are checking anything that is custom to see if anything is interfering with the default behaviour of ckanext-showcase at the moment. It is a little strange to me that it is possible to re-index showcases that we create on the new version, but none of the showcases that we import via db-dump. And even though the base-type of showcase and dataset is the same, the indexing only fails for our showcases.
If you have any other ideas, kindly let us know. Will post any updates, that we find, as well.