nikivancic
This last guess turns to be correct. I only need a single Login redirect URI, just as explained in your article (<a href="https://nilavema.space/wp-login.php" rel="nofollow noopener" title="https://nilavema.space/wp-login.php">https://nilavema.space/wp-l...</a>). My problem was caused by using the bad Org URL which has to be defined in the php.ini file - I remembered that the “base” should be “oktapreview.com”, while today I have to use dev-nnnn.okta.com format).
Let me also add that your plugin does not handle the logout action - so the user needs to type in the <a href="http://example.com/wp-login.php?action=logout" rel="nofollow noopener" title="http://example.com/wp-login.php?action=logout">http://example.com/wp-login...</a> URL in order to logout (see “https://www.wpbeginner.com/…”) for details. The default WP authentication adds this “link” to the menu.
Sorry for such verbose post and thanks for your plugin related work