certbot renew / certonly dash error : 'ascii' codec can't decode byte

I indeeed had a non ascii character in that file, the useful command for such problems is :

grep -nRP '[\x80-\xFF]' /etc/nginx

Where /etc/nginx is the directory/file you want to look for a non ascii character. And where [\x80-\xFF] is the range (in this case non-ascii) you want to look for.