I consider myself an amateur and the following is based on my limited understanding. I also live in Germany, but copyright questions would probably cross borders anyway.
Let's assume I developed and released my (yet to be created) application for commercial purposes and used the LGPL version of QT and linked to it dynamically. Complying to the LGPL licence should be possible without releasing the code of my application core. Futhermore I decide that the LGPL version of QT is too restrictive (e.g. no static linking) and subscribe to use the commercial version QT. Any releases of my application would then be without QT source code and statically linked.
The only problem I see here is that statement in the faq entry 2.13:
If I have started development of a project using the open source version (LGPL), can I later purchase a commercial version of Qt and move my code under that license?
This is not permitted without written consent from The Qt Company. If you have already started the development with an open-source version of Qt, please contact The Qt Company to resolve the issue. If you are unsure of which license or version to use when you start development, we recommend you contact The Qt Company to advise you on the best choice based on your development needs.
Can the QT Company really deny using the commercial version of QT in combination with my propriatary application core code? I read (part of) the licence text, but I cannot answer this question specifically. What really baffles me is that even when using the LGPL version of QT (as a library) I retain the exclusive rights to the application core and never have to release the source under any licence (when properly done, of course). To rephrase: In what way can the commercial licence force me to obtain my own source code not using the LGPL version of QT?