There is always the silly typo (which the developer alone doesn't see) is there to humble you. I had unintentionally defined a new macro (what else by cut'n'paste of that single line and blind replacement of EXPORT with IMPORT) for the clients. After you pointed out, changing the typo as follows fixes it:
Qt Code: 
  1. // change the following
  2. #define myWidget_DLL_IMPORT Q_DECL_IMPORT
  3. // to redefining the "real" export macro as
  4. #define myWidget_DLL_EXPORT Q_DECL_IMPORT
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Thanks for your help!

I guess all the clean pure C++ code that I gained by using CMAKE_WINDOWS_EXPORT_ALL_SYMBOLS variable is gone now. At least for the DLLs that emit signals ...