Prosit seems to be working for me.

A couple of years ago there was someone else who was getting the "failed to connect to all addresses" error:
https://skyline.ms/announcements/home/support/thread.view?rowId=50286

In that case, I believe the problem was that the person's IT department had set things up so that it was not possible for Skyline to connect to the server that it needed to.
Skyline needs to connect to port 8500 on the server proteomicsdb.org

"8500" is not a common port number, and it is possible that your IT department is blocking it.

One thing that you could try is enabling the WIFI hotspot on your cell phone, and see whether Prosit works when your computer connects through your cell phone's WIFI instead of the computer's regular internet connection.
-- Nick