Redirect URL

What should the Redirect URL be? If I use the Callback URL, then...

• the first time a user attempts to "Sign in via Meetup", the authorisation pop-up seems to work (and Meetup.com shows the site as an authorised application). But once the authorisation window closes, the main window redirects to http://[domain]/meetup_token and reports:

  Error

  MeetupUnauthorizedRequestException: <p><b>401 HTTP Error:</b> Error not authorized. Please check your Meetup API credentials</p> in MeetupConnection->get() (line 50 of /[path]/sites/all/libraries/meetup/MeetupConnection.class.php).

  The website encountered an unexpected error. Please try again later.

  Plus the recent log messages include the following...

  1. 
     TYPE php
     DATE Friday, June 26, 2015 - 01:01
     USER Anonymous (not verified)
     LOCATION http://[domain]/meetup/redirect
     REFERRER http://[domain]/
     MESSAGE Notice: Undefined variable: _SESSION in meetup_login_redirect() (line 94 of /[path]/sites/all/modules/meetup_login/meetup_login.pages.inc).
     SEVERITY notice

  2. 
     TYPE php
     DATE Friday, June 26, 2015 - 01:02
     USER Anonymous (not verified)
     LOCATION http://[domain]/meetup_token
     REFERRER http://[domain]/meetup_token
     MESSAGE MeetupUnauthorizedRequestException: <p><b>401 HTTP Error:</b> Error not authorized. Please check your Meetup API credentials</p> in MeetupConnection->get() (line 50 of /[path]/sites/all/libraries/meetup/MeetupConnection.class.php).
     SEVERITY error

  3. 
     TYPE php
     DATE Friday, June 26, 2015 - 01:02
     USER Anonymous (not verified)
     LOCATION http://[domain]/meetup_token
     REFERRER
     MESSAGE MeetupUnauthorizedRequestException: <p><b>401 HTTP Error:</b> Error not authorized. Please check your Meetup API credentials</p> in MeetupConnection->get() (line 50 of /[path]/sites/all/libraries/meetup/MeetupConnection.class.php).
     SEVERITY error

• On subsequent attempts, the authorisation window opens and closes too quickly to see its content (other than to recognise it contains an error message). But the main window reports:
  

  Error

  MeetupUnauthorizedRequestException: <p>&ltb>401 HTTP Error:</b> Error not authorized. Please check your Meetup API credentials</p> in MeetupConnection->get() (line 50 of /[path]/sites/all/libraries/meetup/MeetupConnection.class.php).

  The website encountered an unexpected error. Please try again later.

  And the the recent log messages include the following...

  1. TYPE php
     DATE Friday, June 26, 2015 - 01:21
     USER Anonymous (not verified)
     LOCATION http://[domain]/meetup_token
     REFERRER http://[domain]/
     MESSAGE MeetupUnauthorizedRequestException: <p>&ltb>401 HTTP Error:</b> Error not authorized. Please check your Meetup API credentials</p> in MeetupConnection->get() (line 50 of /[path]/sites/all/libraries/meetup/MeetupConnection.class.php).
     SEVERITY error

  2. 
     TYPE php
     DATE Friday, June 26, 2015 - 01:21
     USER Anonymous (not verified)
     LOCATION http://[domain]/meetup_token
     REFERRER http://[domain]/meetup_token
     MESSAGE MeetupUnauthorizedRequestException: <p>&ltb>401 HTTP Error:</b> Error not authorized. Please check your Meetup API credentials</p> in MeetupConnection->get() (line 50 of /[path]/sites/all/libraries/meetup/MeetupConnection.class.php).
     SEVERITY error

  3. 
     TYPE php
     DATE Friday, June 26, 2015 - 01:21
     USER Anonymous (not verified)
     LOCATION http://[domain]/meetup_token
     REFERRER http://[domain]/
     MESSAGE MeetupUnauthorizedRequestException: <p><b>401 HTTP Error:</b> Error not authorized. Please check your Meetup API credentials</p> in MeetupConnection->get() (line 50 of /[path]/sites/all/libraries/meetup/MeetupConnection.class.php).
     SEVERITY error

Whereas if I try the home page URL, then...

• 
  the authorisation window reports:

  Invalid Authorization request

  If you were directed here by another application, contact its developer

  And the recent log messages include the following:

  1. TYPE php
     DATE Friday, June 26, 2015 - 01:29
     USER Anonymous (not verified)
     LOCATION http://[domain]/meetup/redirect
     REFERRER http://[domain]/
     MESSAGE Notice: Undefined variable: _SESSION in meetup_login_redirect() (line 94 of /[path]/sites/all/modules/meetup_login/meetup_login.pages.inc).