Once you delete the entries in Project Settings > Section Types > Default Types by Structure, the only remaining entry will be a folder document type level entry. Clicking on the “+” - Button will only create additional folder levels. There seems to be no way to get a document level back in there.
The same things happens, when you delete all entries except for a single document level. Form this point on no more folder levels can be created, only document levels:
Maybe there should be some “Reset”-Button to be able to start from the initially available level structure
Windows7 / 220.127.116.11 (32-bit)