It was actually due to the JCH plugin mentioned, but I think that an error arose here after the last Joomla update or another component.
I have now opened the second page again by excluding the contact forms from the optimization by JCH. Please see: https://www.monkey-security.de/kontakt/bewerbungsformular.
I will import the first page and then see why it is here, because the same plugins and components are used there as on the second page.
However, I still noticed a problem with the Admin Tool, because a pop-up does not open during the cleaning process, but the entire page is used for this and nothing happens, here is a short video from the second page: https://gyazo.com/0abcb87da0111e9d6842d760ab38c43e.
I also have a problem with the backups in Akeeba Backup, they don't work anymore, please see: https://gyazo.com/75ff76ec44cd09d1545069d3ee5a2c72
I have tested both with and without deactivated JCH Plugin.
Thx for your answer in advance.