Strange,
[PLESK_17_PHP71]
name=PHP 7.1 for Plesk 17
baseurl=http://autoinstall-dev.plesk.com/PHP71_17/dist-rpm-CentOS-7-x86_64/
enabled=1
gpgcheck=1
was missing, I added it using nano, I'l try again to see if it resolves the issue.
Edit: issue resolved, thanks for your very fast help!