I had this issue where I got "Access Blocked: This app's request is invalid" when I wanted to edit the personal credentials. This was fixed by Dataiku support in this ticket.
This fixed it, but we needed to edit the source code of the plugin (and it was not in line with the documentation).
We have tested DSS with enabled ssl and FQDN and it turns that you will need to use webapp credentials to make it work. (So that's instead of the desktop app)
You will have to add dip/api/oauth2-callback to the end of redirect url. (so the URL to your dataiku instance + "/dip/api/oauth2-callback")
I had this issue where I got "Access Blocked: This app's request is invalid" when I wanted to edit the personal credentials. This was fixed by Dataiku support in this ticket.
This fixed it, but we needed to edit the source code of the plugin (and it was not in line with the documentation).