[TYPO3-commerce] No PaymentArticle in basket

[dk]

Hi everyone.

This is another message concerning the odd "MAIN:FATAL! No payment 
possible because I don't know how to handle it! (class not set!:, file 
not found!:)" error.

I included one custom payment method weeks ago which actually worked out 
just fine until now. Now - no matter which payment method I choose in my 
basket - I get this annoying error every time I try to finish my checkout.
I investigated a bit and found out that 
tx_commerce_pi3->getPaymentType() can't find any payment method having 
been added to the basket and for that reason throwing this error.

So - why is no payment method being added to the basket at all?

Regards,

D.