Problem using a CDialog derived class in a DLL

Dear co-sufferers,

please consider the following problem:

I have a class cNWCLanguageDialog which is derived from CDialog
and resides in a DLL.
Now I want to derive a class cDelFilesDlg (which resides in a different DLL)
from this class.

Step 1: I use class wizard to derive cDelFilesDlg from CDialog
--> everything compiles, links, works.

Step 2: In cDelFilesDlg.cpp and cDelFilesDlg.h, I replace all occurrences
of CDialog by cNWCLanguageDialog
--> everything compiles but doesn't link.

The error msg I get is (translated from German):

cDelFilesDlg.obj : error LNK2001: Unresolved external symbol
"protected: static struct AFX_MSGMAP const cNWCLanguageDialog::messageMap"
(?messageMap@cNWCLanguageDialog@@1UAFX_MSGMAP@@B)

My guess: the problem arises because the two classes
cDelFilesDlg and cNWCLanguageDialog are
in different DLLs. (Reason: I have another class derived
from cNWCLanguageDialog which works fine
but is in the same DLL as cNWCLanguageDialog).

cNWCLanguageDialog is marked as AFX_EXT_CLASS,
the .LIB file of its DLL has been used
for linking with the other DLL.

WHAT's going on here???
All suggestions welcome, of course!

Thanks
Norbert