Inherited UserControl

I return to the problem with UserControl in WEC 2013.
As mentioned in another question, To see newly created user control in toolbox it should be located in sepearate project. Ok, this works great.
But I need create user control, inherited from another user control (3rd party library). When I add any reference to another control in my control library (e.g. enum), control and assembly is missing in control list in “Choose Items…” menu and i cannot add control to toolbox. I think, reference manager cannot find referenced assembly. My tries with registry paths [HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\.NETCompactFramework\\AssemblyFoldersEx\MyAssemblies] are unsuccessful.
When I remove reference to another control (enum), all works fine.

Dear @Absolon

We’re doing very little Application development here at Toradex, so we have basically no expierience with UserControls in Visual Studio.
As this topic seems to be neither related to our hardware nor to our SDK, I assume Microsoft should be able to answer your question. It might also be worth to ask the vendor of the 3rd party library. I would expect they saw similar issues with other customers, too.

Best Regards, Andy