If you search for the Oracle error “Io Exception: The Network Adapter could not establish the connection” (ORA-17002) you will find plenty of discussion on the web. It usually comes down to what the message says, that is a network issue. But the error can also be caused by e.g. incompatible jar files. In my case, however, I was trying to access a 11g R2 instance from SQL Developer and during initial setup everything had worked. And local access, also with SQL Developer, worked absolutely fine.
Solution: Replace “simple” DB hostname with the fully qualified one. I am not sure why this changed the behavior. I can think of either a DNS setup issue (although my search list included the domain) or some kind of reverse lookup logic that is being applied. Anyway, I wanted to share this finding and also record it for my own future use.