While it could be related to something proxy, investigate DNS first. Your error indicates a DNS issue and IPv6. From terminal use Goolges DNS servers to query for the IPv6 of download.moodle.org:
dig @188.8.131.52 download.moodle.org AAAA
From server location (Austin, Tx) I get:
download.moodle.org. 300 IN AAAA 2400:cb00:2048:1::8d65:71b3
download.moodle.org. 300 IN AAAA 2400:cb00:2048:1::8d65:70b3
While it's NOT recommended to make this permanent, one could add Google's DNS servers to resolv.conf - list them first before your normal DNS servers.
The TIA - try it again!
Any entries made in resolv.conf could be commented out by placing a # character at the beginning of the line. (if successful and you find you need to do this again, just un-comment the lines).
And remember that CloudFlare is in front of download.moodle.org ... could be a temporary 'hick up' there! :|
'spirit of sharing', Ken