In krita master loading of fill layers always results in a black layer. 2.9.x is working correctly.

It seems that the saving of the fill layers writes out the files in the current locale thanks to QDomElement.
So for me the color of the fill layer is saved as:

<!DOCTYPE params>
<params version="1">
 <param type="color" name="color"><![CDATA[<!DOCTYPE color>
 <color>
  <RGB space="sRGB-elle-V2-g10.icc" g="0,5270923972129822" r="0,5271534323692322" b="6,103608757257462e-05"/>
  </color>
]]></param>
</params>

The behaviour of QString has been changed in this regard. In 4.8 it first tried the current locale and then C locale. In 5.x only the C locale is used to convert to double. Which results in all values being 0.
The attached patch fixes the problem by using available functions to convert the doubles.