Building debug binaries using the def files frozen with a release build produces unusable dlls.
The debug binaries export more symbols, and import some of these.

for example, qDebug is exported from corelib and used by the other libraries in debug mode.
As this symbol doesn't exist in the release mode corelib, it doesn't appear in the def file.

Workaround is to freeze the def files for debug build, but that will cause postlinker errors if you later try to build release.