Multisite subsites error


After update to we 5.1 my multisite had a error om the subsites.
In the frontend everthing is oké but in the admin i can,t edit pages or ADD new pages. Ik get a blank screen. Also i cant acces the theme options.

I have done
Change theme to default
Shut of al plug-ins
Check htacces ( look oké)
Update themes and plug-ins

Nothing helpt.

Also ADD a new subsite isnt working. The CSS is missing.

I grant acces voor wpmudev

Please help me, i really need to have it working normal.