When dealing with plugins and shared libraries you have to know and understand a couple of things - unfortunately the Qt creator is not very helpful with finding out reasons why something fails.
- I recommend to remove all what you did so far and install Qwt with "qmake + make + make install".
- Do a "ldd /usr/local/qwt-6.1.0/plugins/designer/libqwt_designer_plugin.so" and "ldd /usr/bin/qtcreator" and compare if both are resolving the same Qt and Qwt libs.
- Start a shell and do: export "QT_PLUGIN_PATH=/usr/local/qwt-6.1.0/plugins; /usr/bin/qtcreator"
On Linux it doesn't matter which libs have been built in debug or release mode - you can mix like you want to.
Uwe
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