Unable to Access the Partner Center After Disabling Advance Partner Center

If a Partner record has an Advanced Partner Role and Advance Partner feature got disabled, the record switches the Role back to Partner Center but it loses access. To resolve this, the Administrator needs to edit the Partner record and reinstate the access:

1. Navigate to Lists > Relationships > Partners.
2. Edit the Partner record. 
3. System Information tab > Access sublist.
4. Give Access = T.
5. Provide Password and Confirm Password.
6. Click Save. 

Note: The Advanced Partner Access feature can be disabled using Administrator role under Setup > Company > Enable Features > Web Presence.