First, you'll have to enable Magento's own error logging:
- In the admin control panel, navigate to System > Configuration > Developer > Log Settings and set the Enabled option to Yes
- Check the permissions on the /var/log folder inside your Magento installation since that is the place where Magento's log files are stored. It should be set to 777.
- Finally, in the BitPay payment plugin under System > Configuration > Sales > Payment Methods, set the Debug Log setting to Yes and save the changes.
- Now when you view the /var/log folder in your Magento installation directory, it will contain the bitpay debug log file that you can use to troubleshoot any issues.