I cannot finish my order in WooCommerce with PayPal Plus (Sandbox mode), what can be the cause?

Here you can see a step by step description of what I am doing. Any help would be appreciated!

I have tried to place many orders but have not been able to check out. Here you can see the "My Account > Orders" page (the one order above has been completed, for the others, I was not able to pay):

I add products to the cart then click on "Checkout". I enter my credentials and choose PayPal Plus as the payment method, then click on "Place the order".

After that the site seems to be loading, about 10-15 seconds are spent, then a failure appears:

I can see the new order in "My Account" at once:

I can see all the orders on my "Admin Dashboard" as well, but they are somehow not completed, they seem to be in "on hold" status:

So I have to log in as a customer, go to "My Account > Orders" and have to manually complete the order by clicking on "Pay (=Bezahlen)". After clicking on this link, I can choose the payment method - PayPal Plus - and it redirects me to the checkout page of PayPal.

What could be the cause of my problem and how could I solve it?