I found a solution. It´s the version of jquery. With the plugin jQuery Version Control I go back to an older jQuery Version and the calender works again although I`ve installed wordpress 5.6.
Select your desired jQuery version: jQuery Core 2.2.4
Select your desired jQuery Migrate version: jQuery Migrate 3.3.1
This works for me but I hope the plugin will get an update so that the calender works with the actual version of jQuery too.
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