Thank's for getting back. Weirdly we did successfully create the tables, but since this doesn't work without partitioning, the table would eventually end up in a weird state where the iceberg metadata and the delta metadata created alongside would deviate. 
We did work around this now by creating the table via databricks API SQL endpoint. This seems to work.

Although I have to say that an iceberg table without partitioning is not worth much because partition-/file-pruning won't be possible.