So, quick recap: I decided to pick up Rights of Man for EU IV and Reaper's Due, Sunset Invasion and The Republic for CK II. Tried on Friday using credit card and got the dreaded Payment Refused message. Tried using PayPal and only got a redirect with no error message. Filled a support request, learned about the 24 hour blacklist. Tried again on Saturday with a different credit card and got nowhere. On Sunday around 18 CET, which is 19 local for me, decided to give it one last show using my PayPal. This time it worked but I noticed that the amount was higher than i remembered from Friday. So I dropped everything from the cart and took another look. The Rights of Man expansion for EU IV was 25% off, as it was at all times during the sale. But the content I wanted for CK II was full price, despite the fact that for the three mentioned above the discounts should have been somewhere between 25% and 50%, if I remember correctly. I used my previous support ticked to mention this but got nowhere again. So in the end I picked up the Rights of Man expansion and will probably get the rest in the future but probably not via the Paradox Store, which seems to have a few issues.
LATER EDIT: Well, the problem was that it only ended for some stuff. A lot of EU IV content was still correctly discounted on the 20th. A very small number of other minor DLC for other games also was discounted for the entire duration.
The fact that the sale did not work correctly should have been noticed by the people from support, who told me basically two times that it was all good and that the discounts were there.