Have you checked or compared your .JSON to your old one? Just to see if there are some tags missing?Languague file is ok, work with previous version, i cleared cache and nothing
Any other suggestion?
The admin site lost all the stylish maybe some file lost the url of css files but i don't know nothing about php
Don't tell me you don't have a backup?
Also, try comparing the number of files from before and after the update.
You know Basic troubleshooting