@ambershark That seems to be the case, for some reason the tool copied the dlls from the MinGW-Folder instead of the MSVC ... every single dll was the wrong one.

That also explains, why no SQL-Driver was loaded. Wrong plugin-folder as well.

@hskoglund said in How to correctly deploy a modular Qt-Application on windows? (Contains: can not display svg-files in qml):

You could check the list of directories from where Qt tries to load its plugins by calling QCoreApplication::libraryPaths()

That looks like a cunning plan :) <insert Blackadder reference here>

I played around with this information a bit and made the following observations:

Dumping the library paths in the constructor of my module indeed lists only the module directory, but not the application directory. So I guess you mean the relative path when you say

Your module_ui.dll inherits the search path from the .exe for .dlls

Adding the application folder to the library search path within the module constructor solves the issue without the need for double-deployment

Thanks a lot, you brought me on the right track.