This error occurs immediately upon starting Conga Composer and it's caused by the "Lock IP Address" setting being enabled. It needs to be disabled for hosted AppExchange applications.
WHAT TO DO
- Go to Salesforce Setup > Administration Setup > Security Controls > Session Settings.
- Ensure that the "Lock sessions to the IP address from which they originated" field is not checked.
- Click Save.
- IMPORTANT! You must RESTART all instances of the browser for the setting to take effect.