Wordpress issue - WPMU Dashboard does not show, CSS not loaded.

Hi, worldbest Support team,

I am facing strange issues on the mentioned website:
1. WPMU Dashboard is active, but invisible from the menu
2. after installing my FAB-FOUR (Dashboard, Snapshot, Defender, Smush), Themes CSS does not load, strange enough this happens only in FF & Chrome, Safari looks perfect.
3. Installing Hummingbird leads to a error message
4. deactivating /reactivating the plugins did not solve the problem either (although in 11 out of 10 cases they are responsible )

I have tried reinstalling Wordpress and the Theme (Enfold, both in its latest version), but this did not help either. So, as I am without an idea now, your kind help is soooo welcome, ThX.

Best regards, Sebastian.