If I understand your description correctly, you've decided you would like to change your research agenda from your initial proposal, giving a more general research agenda. It sounds like there is some disagreement among the academics you mention as to what is an appropriate scope and focus for your research project, and whether or not they support this change. If I understand your description correctly, there is a professor and a postdoc who agree with your more general research goals, but your primary supervisor is encouraging you to focus on a particular applied part of this broader agenda, which was your initial proposal.
A PhD candidature is a research-training program, and the proof of development is generally the ability to create publishable research outputs (e.g., academic papers conference papers, etc.). Nothing in your post really sets out a plan for how you want to achieve this within your research vision, so that is something you will need to think about. If you focus on the narrower applied project at first, and are able to successfully publish research on that topic, it may assist you in developing towards completion of your candidature. Similarly, if you can publish research in the broader areas of interest to you, that may also assist you in developing towards completion of your candidature. The best way to proceed depends on a complicated calculus of risk, reward, motivation, etc., but you will need to formulate a pathway forward where you can publish a sufficient amount of research to meet the requirements for successful completion of your candidature.
It sounds like you have some general motivation issues with regard to the research, and you also appear to have some unrealistic ideas about the expectations for a PhD candidature. Particularly because of the latter, I recommend that you do not follow your own instincts too closely, without significant counsel from the academics supervising you. You should have a discussion with your supervisor and try to formulate a program where you will publish research but where you are also heading towards areas of broad long-term interest to you. Motivation issues are tricky in graduate-level education --- on the one hand, we ideally want students to pursue research activities that they find interesting, but on the other hand they also need to develop some stoicism in relation to the slow pace of development of work and the necessity to work on problems of less interest sometimes.
In regard to formulating a way forward, I would strongly caution against your assumption that only the people who are being encouraging of your proposal to change your research program are being helpful --- sometimes it is also helpful to discourage approaches that may not bear fruit. Your supervisor might be of the view that there is a greater prospect of research publication in your initial applied topic, or might have some other reasons for suggesting you focus on that part at present.
I’ve the impression that he talks behind my back to my other supervisor and the main professor.
That is part of his job; when supervising a doctoral candidate it is expected that you will confer with other supervisors and possibly even other professors outside the supervisory group. There is certainly no expectation that all these discussions would be limited to situations where the student is present. Bouncing ideas and opinions off other academics is a helpful way for a supervisor to test whether they are providing good advice to the student and it is a useful mechanism for general alignment of best practice in supervision (and it is also sometimes just a helpful way to vent to colleagues about day-to-day work frustrations).
