If the access_token key is removed from the ezx_oahth_client_consumer_user table everytime you try to login into service portal using "Service portal" tab in the admin interface you'll get

An exception occurred during oauht server negotiation

Invalid signature

Steps:

1. Clean instalation
2. Register in service portal
3. Run sync_network and monitor cronjobs
4. Truncate ezx_oauth_client_consumer_user table
5. Login into admin interface
6. Click on "Service Portal" tab
7. Click on "here" link
8. Click on "Continue" button