Trying to upgrade to 7.11.12, getting ImapHandlerFactory.php error

[pgr]

Make sure you read the advice from the Release Notes:

Debugging steps for use in upgrade version prior to 7.11.11 and 7.10.23

Known issues with the upgrade process fixed as part of this release will unfortunately not take effect until the next upgrade cycle. Therefore it is vital that if you encounter any issue while installing that you review and follow the recommended process as noted below: 1: The screen becomes blank and stops loading. Symptoms Only visible are the background colours for the header and left-sidebar. The page has stopped loading, and the refresh icon is stationary. The loading centre box has d…

And also, always delete the folder cache/upgrades/temp before each upgrade attempt.