I would suggest you 1) clear the settings (check the knowledge base) and 2) rename the plugin directory and see if that works.
Try one thing at a time.
I´m going to take a wild guess and say you have to re-install Cubase if you want to downgrade, but I havn’t done it myself.
It is however tempting because I’ve had several issues myself after updating (blacklisted plugins, freeze ups, pop-ups with “a serious error occurred” etc. Cubase have never been very stable imho so if you find a version that´s mostly working for you then stick with it.