Static Picture ID in Post Command

Hello Forum,

I’ve encountered the following problem:
Using a picture from the Mediathek (media library) will fail due to a problem with the code that is executed when pressing the “Beitragsbild festlegen” (set article picture) button. With a man in the middle attack of this code, I have figured out that the Picute ID in the code is always set to “-1”. When I modify the Picutre ID before the execution of the command the picture will be used correctly in the article afterwards. So the problem is definitly the fact that the Picture ID is not parsed correctly in the code behind the button press.
My problem now is, to find out where in the word press source code this code for the button is generated and why is it set static to -1.

To complete your picture of the wordpress environment the following plug ins are also installed. And the problem exists not only with Version 4.94 but also with older versions like 4.7x

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BR Chris