IntelliJ giving 'internal error' for some (most!) quick-documentation invocations. Am I somehow mis-configured?

Using the Scala plugin, quick-documentation is not working for many cases where (I expect) it should, and reporting 'Cannot fetch remote documentation: internal error'

I am using a default install on Windows 8.1, with a project created by the SBT project creation wizard with no non-default configuration (at least conciously).

Here's a completely trivial example created by entering a minimal 'Hello world' app object into the empty project framework created by the Scala plugin project creation (SBT) inside a package called 'hello':


If I select the 'App' from the 'extends' clause and invoke quick docs (ctrl-Q) I get an appropriate docs popup:

so far, so good.  If I now click on the link to 'DelayedInit' however the result is this:

Looking in the IntelliJ log I see a long stack dump that begins:

2015-09-12 15:41:32,990 [  18712]   INFO - mentation.DocumentationManager -  
java.lang.NullPointerException
 at org.jetbrains.sbt.editor.documentationProvider.SbtDocumentationProvider.org$jetbrains$sbt$editor$documentationProvider$SbtDocumentationProvider$$extractDoc(SbtDocumentationProvider.scala:33)
 at org.jetbrains.sbt.editor.documentationProvider.SbtDocumentationProvider$$anonfun$generateDoc$1.apply(SbtDocumentationProvider.scala:29)

Probably related to whatever causes this (but I'm not sure it is), the presence of quick-doc info at all seems very erratic.  To illustrate here's a slightly perversely complicated hello world variant:

object Hello extends App {
  val someList = List("Hello world")

  def listHead(ls: List[String]): String = {
    ls.headOption.getOrElse("hmm")
  }

  println(listHead(someList))
}

Now if I select 'List' from either the val assignment line (the one defining 'someList') or the type specifier for the 'ls' param of the function 'listHead' and attempt to get quick doc:

Here I get some basic type-info, which doesn't include any JavaDoc related to the List class at all (which I think I should - contrast to what I got for 'App' in the earlier image).  If I try to click through the 'List' hyperlink I get the original 'internal error' issue.

I'm not sure if something is mis-configured, or I am simply misunderstanding something about quick-doc.  Can anyone shed any light on this for me?