Error when changing materials in source file of linked objects Maybe I'm overlooking something but I have a problem and don't want to screw up my files ;-)

I created my Assets for a scene in one file, put the objects into groups (each object got its own) and am then linking the group in my scene builder file. 

Everything works great that far. 

I now wanted to change some materials on a few of the objects that I linked, unifying them basically.

Therefore I changed the assigned material in my asset source file. 

When reloading the scene builder file now I get errors corresponding to the materials changed:
Lib: Material: 'InsertMaterialName' is missing from  'AssetSourceFilePath', parent ''

So it seems to still point to the old Material even though I changed it in my source file. 

Does anyone know why this happens and how to fix this? 

Help is greatly apprectiated. 

Cheers
Daniel 

Edit: 
Removing the Materials from the scene outliner -> Blender File -> mySourceAssetFile.blend
removes the errors. Though I really would like to know where this error comes from and how to "really" fix it!
This is normal behavior and expected as far as I know.

The destination file where you linked from saves both the group you linked to along with the data contained in that group. That means objects, materials, image textures, animations, particles. In doing so it saves the link to said group but also a reference to the internal data.

If you change the linked file, when reopening the master those references will be missing from the group, and hence the warning you get. This is however expected, and if you save the master file, close blender and reopen, the broken references should be gone and so should the warning too.