Hi @ccome, so I just gave this a go with the settings you have provided and did not run into the problem you have described (my own Salesforce login domain would also be something like foo.my.salesforce.com here). I was also using a local domain for n8n only configured in my host file which is not reachable from the outside.
One noticeable difference was that for me it never said just “Full access (full)” in the “Selected OAuth Scopes” field. Salesforce instead listed all of them for me:
So maybe you could try manually adding all scopes (instead of picking just Full access (full)) for the app, just to double-check if this changes the behavior?
On a side note, Salesforce said
Changes can take up to 10 minutes to take effect. whenever I updated the configuration or created a new app in its Manage Connected Apps screen. In reality it was much more than 10 minutes for me and I got a lot of different errors during the wait (form redirect_uri_mismatch to very generic errors).