I ran the installation from the cli again which resulted in the same error. It seems that yunohost is still fetching etherpad 1.6.5 instead of 1.7.0. Is there a way to force download of 1.7.0 ?
I used the correction suggested by @tierce : Etherpad_mypads - 502 Bad Gateway - Migration Yunohost 2.7 > 3.1
which worked.
Installation successful !