[solved] QwtDoubleInterval in Dll not found

I have an older VisualStudio Project using qwt 5.2.x

I compiled the project with Visual Studio 2008. I copied the qwt sources to the project, so that the correct headers are used.
Then I compiled qwt itself with VS and put the qt dlls and the compiled qwt dll to the binary folder containing the executable.

However now I get the failure that QwtDoubleInterval in is not found in the dll.

EDIT:

I accidentially copied the sources of qwt to the wrong project. Now I synced all sources and everything compiles, links and starts as expected.