Irina,
Thanks for the reply.
Fixed it.
Here’s what happened.
I have both v3 and v4 in my toolbox but I only use WPF4.
By mistake, I dragged some v3 controls (calendar, time editor & new Masked Text Box) into my project instead of v4.
That got the C1.WPF mixed in.
Interesting behavior: Masked Text Box caused the error only when I gave x:Name a value.
Anyway, I went through the project and removed all references / usages / namespaces for v3 and replaced them with v4.
Everything works fine now.