unexpected error

Hi,
I have just installed the plug-in and always run on the same error when editing an existing post or when creating a new one and pasting plane text. It says

“Im Editor ist ein unerwarteter Fehler aufgetreten.” With ihre buttons that all have no action that I can see:
“Wiederherstellung versuchen”
“Beitragstext kopieren”
“Kopierfehler”

In English that means
“An unexpected error occured within the editor”
“Try to recover”
“copy post text ”
“error on copy”
sorry for the bad translation.

So I have no clue how to continue.
WP version is 4.9.4
Any ideas?
Thx and cheers, Dietmar