I was trying to use a static class of attached properties that I would use inside my WPF control templates, so that every time I need to change them I would only use a setter, not copy paste the entire template and make minute changes. It worked great and I recommend this solution to everybody. So after the refactoring of my control I was shocked to see that in the production application, all my attached properties were throwing binding errors!
The solution was to explicitly use the Path= syntax in the bindings.
The strange thing is that in the test application everything worked great, so what went different? Apparently the problem is that IXamlTypeResolver (used in PropertyPath) is not finding namespaces unless already registered, as explained here by Rob Relyea. The dude with that Lovecraftian name is the program manager for the WPF & Xaml Language Team.
So if one uses the namespace of the attached property before in some other context or by using the verbose syntax before, it works even with the Path keyword omitted. Not really sure why the verbose syntax works differently though.