GET requests work but not other requests

Hello,

I am using nginx as proxy for my ghost blog. After solving a lot of issues with the ghost help forum I came to a point where these guys told me it is not a ghost related issue, it seems to be a pure nginx topic.

I have no idea what could be the issue but I received the hint to ask you for this issue:

the fact that GET requests work but not other requests

I am able to log in, do some stuff but if I try to save a new blog entry I am receiving a error message.
I hope you can help me out because I am doing it just to follow the wish of my wife to try blogging.

Thanks!