Hi,
I had the same problem as you and this is the solution it worked for me:
1.- Right click on the project name and select "Unload Project".
2.- Right click on the project name and select "Edit <<Project name>>"
3.- Search the resource file name, let say MyResource. You will find something like:
<Content Include="Skins\MyResource.xaml">
<SubType>Designer</SubType>
<Generator>MSBuild:Compile</Generator>
<CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
</Content>
4.- Replace it with:
<Content Include="Skins\MyResource.xaml">
<CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
<Generator>MSBuild:Compile</Generator>
<SubType>Designer</SubType>
<Generator>MSBuild:Compile</Generator>
<SubType>Designer</SubType>
</Content>
5.- Close the project file.
6.- Right click on the project name and select "Reload project".
7.- Cleck the problem is gone and Visual Studio can load resources at Design time.
I did not check if the solution comes for the change in the order of the XML tags or it comes due to the double "Generator" and "SubType" tags. I had no time to find it out.
I hope this may help some one.
Benjamin