Ok. I do not know what has happened, but suddenly it works!
However, to provide some more infos on people trying to achieve the same thing:
I first got ‘Error 500’ with no output.
My current (working) setup looks like this:
The blade template is located in
In my functions.php I echo the content like this:
echo \App\template('ajax.form', ['var' => $variable]);
It works as expected: I have the $var in my template available. I still can not understand why it suddenly works and neither can I reproduce the error, but I’ll keep you updated as soon as the problem reoccurs.