So its just like the title says. Here's the scoop:

When loading Skyrim.esm and Skyrim.esm + Update.esm, I get an error message:

In response to this I either hit "RETRY" or "IGNORE". Both return the same result: These Master files load without further incident.

However, if I attempt to load more than one DLC master file, like Dragonborn.esm + Dawnguard.esm or Dragonborn.esm + Hearthfire.esm, I am prompted with the message:

What's with this? Why can't I have two or more dlcs as parent masters? Anyway, it goes even further:

Unable to load more than one DLC as a master I attempt to only load one. At first, I get the same message as before:

I hit either "RETRY" or "IGNORE".

Then I get this error message with one variation between them, depending on the dlc I attempted to load:

The variation is the name of the DLC. It is either Dawnguard, Dragonborn, or Hearthfire.

I hit either "YES" or "YES TO ALL". Both return the same result. The Creation Kit stops working and crashes to desktop.

What in Oblivion is going on here?

You need to add the BSA files of the DLCs to the SkyrimEditor.ini file in order to load the master files. Alternatively you can extract the strings from each DLC's BSA files.

Before

SResourceArchiveList2=Skyrim - Shaders.bsa, Update.bsa

After

SResourceArchiveList2=Skyrim - Shaders.bsa, Update.bsa, Dawnguard.bsa, HearthFires.bsa, Dragonborn.bsa

In order to load multiple master files you will need to add the following line under the General section of SkyrimEditor.ini:

bAllowMultipleMasterLoads=1