The tag does not exist in XML namespace

Error:
The tag 'XXX' does not exist in XML namespace 'clr-namespace:YYY;assembly=YYY'. Line zz Position zz.

Solution:
I know, it's really weird, but the solution is simple. Just change
FROM:
xmlns:ZZZ="clr-namespace:YYY;assembly=YYY"
TO:
xmlns:ZZZ="clr-namespace:YYY;assembly="
leave empty value for assembly=