I agree.  I'd say reinstall the old Epson driver.  If QU loads OK after that, just change the printer in QU to the new printer and close QU and you can then try deleting the old driver again.  I haven't seen this problem before: it's normally fine to delete a driver.  QU will normally just give you a message "Unable to load driver settings for <your old printer>" and it'll just switch to another printer on the list.  I just tried it by installing the Epson P800 driver, setting up QU to use it, then deleting the P800 driver.  The next time you start QU, it just tells you it can't load settings for the P800 and it switches automatically to my Canon Pro-100 without any fuss.  So I'm not sure what happened in your case because it is designed to detect-and-correct when a driver is no longer found on startup.

Regards,
Mike