so maybe you guys want to have an update. Because I want to tell you, that I didn´t blowed that off with shanghai, THEY did.
I had a really nice contact with the person from the University, and she was very convinced that I can enroll at their University, because they offer this a lot for foreigners. And that I could enroll as her personal doctoral student and also would be able to do fieldwork in Yunnan, she already asked the secretary. So it already was a YES from the University side, it was just about the duration of time. So I was REALLY happy, last week on tuesday she told me. Yesterday I still didn´t hear back from her so I asked her what the secretary said. And she just answered she will not be able to host my visit. nothing more. no explanation. I asked why, because it already seemed safe, no answer.

So right now, I am super frustrated, because she already said yes and suddenly a no without any explanation. I don´t understand anything right now.

Dealing with a foreigner for the locals means getting their head around difficult cases. Admin staff is often undersized and they work overtime.
The westerners' approach is to make sure of all impediments before coming here, and to try starting solving them remotely.
By going our way, the locals understand even more that our case will give them headaches. This teaches us that we employ a bad strategy.
You should just try to come here first (but I agree you need some proper visa and some GENERAL written agreement), and later push your way through the details, face to face. So that headache you will reciprocal give to each other, will be more bearable and less likely to incur into an eventual "no go".

The is bureaucracy with Chinese characteristics. The person who originally said yes, did not have the decision making power, and she was overruled. It may be an internal policy issue, it may be a subjective matter, it may be based on your application. Whatever the reason, trying to find out the actual reason could well be a fruitless endeavor. It is quiet normal here for people in authority not to give a reason, and they won't budge. The woman probably cannot answer you, either because she does not know, or she is not at liberty to tell you. It is pointless embarrassing her further by pushing for a reason, because that is likely all that can really be achieved within the culture (face). It is also quiet possible that you would not be given the real reason, also normal here.
Sorry for your disappointment. The best thing may just be to move on to plan B.

I also wrote my german supervisor, because he knows her personally and he maybe have others contacts that could help me, but also, still no answer. It´s a waiting game which is driving me nuts. I can´t do anything about it, just wait, feeling happy when I have positive mails and then again feeling disappointed because the former yes always turns into a no.

yes can mean no, no can mean maybe, nothing is certain until it actually happens, this is what I have learned from 6.5 years working here. You learn to be flexible!

I should learn that by heart. And I always tell myself that I shouldn´t have such big hopes. It was just after two months of trying without any success I received her mails with an actual YES and it will not be a problem at all .. yes, maybe I was just too naive thinking now it will actually work and I finally have a plan and can totally focus on how I want to do my fieldwork.

I have to agree with everything @Haali said above. My experience too.

I will wait for my supervisors response. I still don´t know what he is able to do, but I really hope he can help me with that, as he was in China quite often and had a lot of doctoral students went to China and come from China ...

So, I thought maybe I can give you an update now, as I there were so many of you who gave me really good advices. I don´t know if you are interested.
I had a really nice contact with the Yunnan normal University, but as I have bad luck all the time, they will not accept visiting scholar, as they had really bad experience last year.
But, I am considering now doing a half-year non degree program at the Yunnan Normal University, trying to find some contacts to Pu'er, but also, trying to get permissions for doing research, trying to do research in Kunming (like comparing the schools of Kunming and Pu'er, or with the countryside). As they will provide me a student visa, I would have half a year handle things in Yunnan, I could go to Pu'er during breaks or for a couple of days (maybe I can make some "official" appointments) and maybe, after the semester I will be able to do fieldwork in Pu'er or have a better option for the visa. But I know, I have to be present in Yunnan now.

This is by far the best option now, it is not perfect or optimal, because still no fieldwork, but I can interact in the University, I can make contacts, I can learn more chinese, I can prepare better and I will just BE there.

I will decide at the end of the week, as I have a really really nice and good contact to the person in charge now.

I guess, it could work like that? I just need to be more active and present and from over here I don´t have any of this.