As the creator is build in release mode you can't load a designer plugin ( or lib ) that has been built in debug mode. So this combination doesn't make any sense at all ( beside you want to debug the creator ).
The message tells you that you are trying to mix debug/release libs. Guess the creator has been built in release mode - so it must be the designer plugin or the Qwt lib that has been built in debug mode.
I recommend to throw away what you have so far and rebuild with the default settings in qwtconfig.pri - what is debug_and_release on Windows.
Uwe
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