The tag 'XXX' does not exist in XML namespace 'clr-namespace:YYY'

Another possible solution to this problem is that you're not using the same version of .Net in your project and your library.

Three fixes to make here:

1. No spaces -> xmlns:converter="clr-namesapce:MyApp.Converters,aseembly=MyApp"
2. No misspellings -> xmlns:converter="clr-namespace:MyApp.Converters,assembly=MyApp"
3. Right delimiters -> xmlns:converter="clr-namespace:MyApp.Converters;assembly=MyApp"

From the the documentation:

Note that the character separating the clr-namespace token from its value is a colon (:) whereas the character separating the assembly token from its value is an equals sign (=). The character to use between these two tokens is a semicolon. Also, do not include any whitespace anywhere in the declaration.

I am exploring as to why this is happening, but if your converter is in the main assembly, removing the assembly= from your xmlns:converters tag should remove that build error.