I noticed one problem. For some weird reason apparently the MySQL database connection was broken (did I reset the password; I don' t remember). For this reason the databases of the contract were not deleted. May be this was the cause that contract delete errored out and did not finish successfullly, but in the front end I did not see any error of this. I should have been warned that something is wrong and the contract should not be deleted.
I don't remember, but I might have deleted the databases manually before deleting the contract.