I think I found a solution - make sure that reTerminal has an internet connection and that either SSH is setup or you have the means to connect to the UART console.
After upgrading and rebooting, the screen will stay blank, yet the OS should be up. Now connect to the console via SSH or UART and reinstall the drivers as outlined here.
The screen should work again after a subsequent reboot. (although I have to say it seems I have a flickering ribbon on the screen now - not sure where it comes from).
I assume that the firmware or kernel upgrade somehow messes with the configuration done by the driver install.
Hope this helps - and hope that this issue gets resolved in the future.