.. and why do I get an error, that Concrete5 is looking for file view.php, when I try to view a Single Page I added?

First read a bit about Single Pages and their relation to view.php here.

So, when ever you add a Single Page yourself, or they are being added by a Package you installed, it will be looking for view.php in your theme directory.

What does it include? Just like your full.php or default.php files, you include some header/footer code, and a single line is needed.

<?php print $innerContent; ?>

That's all!