Thank you very much, tabreturn.
Great, it works.
It was a little bit tricky because I didn’t know exactly what was meant and what I should do.
The path to find the .dll was c:\programs (x86)\processing-3.5.4\modes\java\libraries\serial\library\windows32 and it was difficult for a beginner to find that. 
After deleting the .dll everything works fine.
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