I happened to be on the site today activating the "Post to GPSTC" button for wingdingers, and decided to take yet another look at this. I am still unable to get posting to reliably work all the time without GPSTC occasionally throwing its toys out of the pram and sending users to a login page, even if they are already logged in.
I'm still happy to talk to someone at GPSTC to try and resolve this, but I'm certain the issue is at the other end of the pipe. I've got everything working as well as I can but the current integration is based on documentation from a LONG time ago (www.gpsteamchallenge.com.au/pages/integration), and maybe there is another way to achieve this now. I would prefer to be posting the data to an api endpoint, and getting back a redirection url for the user to go edit the data, but that's just not how it is working right now, and that is part of the issue.
It is annoyingly variable. I was getting the error a lot earlier on, but now I can't seem to get it to happen at all.