I ran into a different cause of this problem, which was unrelated to resx files. I had a class library where AssemblyInfo.cs contained the following:
[assembly: ThemeInfo(
ResourceDictionaryLocation.SourceAssembly,
ResourceDictionaryLocation.SourceAssembly)]
The assembly did not contain any WPF code, theme or Resource dictionaries. I got rid of the exception by removing the ThemeInfo attribute.
I did not get an actual exception, only
A first chance exception of type 'System.Resources.MissingManifestResourceException'.
Viewing exception details, the system was requesting MyAssembly.g.resources
Hope this might be of help to someone else.