Sorry for writing here all that much.
I may have found something though it is bewildering...
In the problematic VM (one not being constructed) I have a property of a type that is defined in another project (external type from here on).
Why I mention this?
I tried to simplify the structure of the windows and put the 2 views (usercontrols+VM) on the same window without tabs or anything
One View-ViewModel combination works fine (this one expects a VM
without external types)
Other View-ViewModel combination does not work (this one expect a VM
with external types)
Now I tried to simplify the situation even more and moved the property that depends on the external type to the main window's VM
When I did that - also the first view-viewmodel combination (and is placed on the main window) that worked before
Now I created the external type as a local type, left the property in the main window's view model and again it works.
This leads me to a conclusion that there is some issue with using the external types - any VM that references an external types (defined elsewhere) do not load with the Catel base code.
Does this make any sense? Should I define something differently?