The source link on the page for http_open/3 (currently https://www.swi-prolog.org/pldoc/doc/_CWD_/pack/mirror/GIT/flux/prolog/indigolog/lib/http_open.pl?show=src#http_open/3) is giving a 403 – seems to trying to open an internal path or something:
You don't have permission to access /home/jan/src/plweb/pack/mirror/GIT/flux/prolog/indigolog/lib/http_open.pl on this server