Try to log out and back in. Server thinks you are not logged when sending that [POST] request.
Must be a corrupt session cookie.
I did and it still throws 'Could not contact the server' upon trying to save the edit.
For shucks and giggles I edited this post from yesterday: https://poal.co/s/ThisDayInHistory/781066 Worked flawlessly. Really weird O.o
Can you copy-paste and PM me the content of the post's content you can't save?
Forgot to mention: I put "Test edit" into yesterday's post (worked) and the same thing into this one (error message).
Done.