Error 403 could be due to accidentally de-authorising the BikeBrain app from the Facebook site. To re-authorize BikeBrain: Use a computer to sign into your Facebook account.
In Facebook, select menu at top right corner > Home > Account Settings, then locate and select Apps from the left hand column. Click the plus symbol (+) to the right hand side of the BioLogic BikeBrain entry to re-authorize BikeBrain.
Then open BikeBrain on your iPhone and go to the Sharing Settings, sign out and sign into Facebook again. When prompted by Facebook, press the “Install” button and “Allow All” button. Then try sharing a trip again.