I'm having the exact same issue and have followed the steps in your reply and I just get an "Invalid site key for the current site." error.
I've tried to see how I can contact support directly on this but all of the support links go round in circles! If you can let me know an email address to contact I can send the details I'm used over so we can resolve it.
Hi, I don't have access to create a full backup of the site currently or to give you FTP access (but can request if needed) but I can send over a WordPress login if that's sufficient?
Please confirm and I'll get it sent over but it's just this invalid site key error that we're having, image attached.
You are using really really outdated Toolset Types plugin.
We publish the Types 3.0 with the new many to many relationships and we have added the number of new features to this major release and that is before almost nine years a go.