@giantrubberduck i am getting the same problem but when i tried to link faforever with steam account from my android based phone it works you can also try to link from your phone.
So, I turned on IPV6 in FAF settings, choose COTURN server Finland (no server was chosen before), turned on experimental debugger. And it started connecting 80-90% times (it's ok for me, because it was around 0% for the last week and much earlier). Maybe this information will help somebody.
Sorry to necro but I just saw this wasn't resolved. I just had this occur on one of my matches (admittedly a short one as my opponent GG'd after an early T1 bomber) and wanted to see if their was a solution or if the issue was narrowed down?
@rama if your testing stuff for modding ect your better looking into setting up a development environment this skips the need to constantly host game in the client.