Ah, thank you so much! I feel like there's a bug somewhere because I've disabled attributes on the Advanced -> Performance page, so I can't even enter any on my product pages.
I just checked if the behavior is different if I turn it on again, but the "-" still shows up. Very odd.
Since I don't use attributes anyway, I've now added an override for PaymentModule.php removing the string marked red in your screenshot and it works correctly now. 
Still would be interesting to find out what might cause the problem because I guess the isset attributes part should only show something if there actually are attributes. I also checked the database and ps_attribute, ps_product_attribute, ps_product_attribute_combination and the others are all emtpy. Weird.