If the curl download from mod.io fails, it will throw an error message failing to parse the empty response as JSON:
But curl has error codes and strings for all the possible connection errors and there is code to catch the error codes.
But only with this bugfix the code becomes active:
This confusing error was reported on the forums too and the bugfix may help avoid unneeded reports:
https://wildfiregames.com/forum/index.php?/topic/24718-mod-downloader-is-not-working/&tab=comments#comment-360171