Discourse now uses the ‘sso provider secrets’ setting for the SSO Provider secret key. To fix the SSO login error, try going to your Discourse Settings page and search for ‘sso’ to see all the SSO settings. I’m assuming that it currently looks something like this:
If so, add your WordPress SSO secret key to the ‘sso provider secrets’ setting:
Enter * into the first field, and your secret into the second field:
Click the 
icon and then click the green checkmark:
Edit: normally the value from the ‘sso secret’ setting would have been automatically transferred to create the ‘sso provider secrets’ setting for you when you updated your Discourse site. I suspect that due to the issues you mention here, that something went wrong during the migration so the value wasn’t transferred.