Well, turned out this was happening with every itch.io game ... after messing around with a bunch of different browsers, restarting windows etc. I tried switching my wifi connection between the two networks I have set up and ... it worked again. Still works after switching back to the other so, no idea what was actually causing this.
edit ... and it's broken again, even when switching between networks as I did last time.
This is also only happening on one computer, have tested on several others on the same networks(s)and all is fine.