Hello Olysak,
Now I know what was going on! I was connecting a sandbox org, so in that case the flow endpoints should be "test.salesforce.com" or include My Domain.
Regarding My Domain, it seems to always work, be it a production org, developer org or sandbox org.
In the following Salesforce help article about OAuth Flow Endpoints, at the end of the article it is stated:
All endpoints require secure HTTP (HTTPS). Instead of using login.salesforce.com, you can also use the My Domain, Experience Cloud site, or test.salesforce.com (sandbox) domains in these endpoints. For hostname, use the My Domain, Experience Cloud site, or custom URL.
https://help.salesforce.com/articleView?id=sf.remoteaccess_oauth_endpoints.htm&type=5
I had another question regarding refresh tokens. At the beginning of January I was having problems with the "Web Location" connector because after a certain timeout (in Salesforce the maximum timeout value is 24 hours), the connected app is disconnected, the connector isn't able to obtain a refresh token, and has to be authorized again in the Connector Manager, see this thread:
However, yesterday I saw that some connectors I authorized weeks ago were still working correctly! Has something changed in the way EasyMorph is managing the refresh tokens?
Thanks a lot,
Roberto