editor toolbar missing after update to mw 1.35

Hello there,
maybe someone can help me out:
I recently updated my Mediawiki from 1.31 to 1.35.14.
 
Problem: After the update the Toolbar on top of the edit window of the WikiEditor is not displayed anymore, when editing a page
 
I did a websearch, but only found a very old resolved Problem of that kind, which hat to do with settings in mediawiki:common.css. But that file seems unchanged on my wiki.
 
Maybe someone has an idea what could cause this problem?
 
Greetings,
Marcus

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