Hey @mattia + @13milliseconds i guess this thread could be interessting to our issue: “Help I tried to trellis up an older project and it’s not working!” and other stories
Github recently changed its ssh key rules. So the old ones, since you installed your trellis project, are not working anymore.
Instead dealing manualy with tokkens, i recommend to do it “the right way” by updating your known_hosts.yml file and re-provision your server.
Finaly it solved my issue.
The output is green (i love green these days), no errors.
I hope this helps you too.
Let me know if you have any questions. 
