Seems related to:

• http://slicer.cdash.org/index.php?project=SlicerPreview&date=2018-04-12#!#Nightly-Packages
• http://slicer.cdash.org/viewBuildError.php?buildid=1240106

Uploading [Slicer-4.9.0-2018-04-11-macosx-amd64.dmg] on [http://slicer.kitware.com/midas3]
CMake Warning at /Volumes/Dashboards/Preview/Slicer-0/CMake/MIDASAPIUploadPackage.cmake:109 (message):
  Upload of package to MIDAS failed: Debug:

    Trying 192.168.120.210...

  TCP_NODELAY set

  Connected to slicer.kitware.com (192.168.120.210) port 80 (#0)

  PUT
  /midas3/api/json?method=midas.slicerpackages.package.upload&token=374nazyC6UEv5FJUPMvaBISFP6OuxOik1VqBNZ7R&revision=27145&os=macosx&arch=amd64&submissiontype=nightly&packagetype=installer&name=Slicer-4.9.0-2018-04-11-macosx-amd64.dmg&checkoutdate=2018-04-11%2021%3A34%3A11%20-0400%20(Wed%2C%2011%20Apr%202018)&productname=Slicer&codebase=Slicer4&release=
  HTTP/1.1

  Host: slicer.kitware.com

  Accept: */*

  Content-Length: 295321678

  Expect: 100-continue

  HTTP/1.1 100 Continue

  [16384 bytes data]

  [...]

  [2044 bytes data]

  Recv failure: Operation timed out

  Closing connection 0

Call Stack (most recent call first):
  /Volumes/Dashboards/Preview/Slicer-0/CMake/SlicerPackageAndUploadTarget.cmake:234 (midas_api_upload_package)

CMake Error at /Volumes/Dashboards/Preview/Slicer-0/CMake/SlicerPackageAndUploadTarget.cmake:250 (message):
  Upload of [Slicer-4.9.0-2018-04-11-macosx-amd64.dmg] failed !

  Check that:

  (1) you have been granted permission to upload

  (2) your email and api key are correct

Next nightly package seems to work:
Slicer-4.9.0-2018-04-11-macosx-amd64, still revision 27145