How do i create an aar archive?

I´m wondering how i could create an aar archive for a third party lib which is available in source format, resources and so on.

Yesterday i tried to create an aar for my MaterialDateTimePicker and i got it working (creating an aar from Android Studio).

Yesterday in the evening and today i´m trying to create more aar´s but failed so far.

Mostly i get the error



Now i guess it was by accident to get it working yesterday...

How do i create an aar archive?

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