Yes that's strange. Must be some kind of glitch on their end.
I do remember a MoneySavingExpert tip - he always said never pay the minimum. If all you can afford is the minimum pament, then set up a direct debit for that same amount. The problem is the minimum payment is a percentage so will keep getting smaller, ensuring that you never pay off the debt. If you set up a DD then at least it doesn't keep getting smaller so it will eventually clear the debt.
I'll just do one last explanation in case this helps before you give up on the CC in YNAB.
In YNAB when you spend on a CC, the program automatically reserves that money for you. I'll share the example of our card this month.
We've made 3 purchases:
Groceries 51.90
Amazon 65.99
Groceries 32.00
That comes to 149.89.
Let's pretend there is a 2,000 balance of debt on the card. So after these three purchases, my card is at -2149.89.
Obviously when I spent money at the supermarket, it came out of the Groceries category. If I do that using a debit card or cash, the 51.90 is gone from the budget - spent. But with a credit card, 51.90 doesn't just get deducted from the groceries category, it moves, to the card payment category (which is right at the top when you first set up; I've moved mine to the bottom).
So my card payment category has 149.89 in it. But because I also have this debt balance to clear, let's say I want to pay 200 off the card. 149.89 is already accounted for, but I need an extra 50.11 to top the payment up to 200.
What you need to do in that case is assign the 50.11 directly to the card payment category, just like you'd assign money into the groceries or electricity category. This should top it up so that the available figure in card payment is 200.
If you have interest to account for instead of purchases, then it works the same (using round numbers to make it easy to see)
Balance of -2000
Check statement to see interest costs you 20
Go to card account page and add interest transaction of 20 (which you budget for like any bill)
Card balance changes to -2020
Assign an extra 30 into card payment category (that already has 20 in it from the interest transaction) topping it up to 50.
Make card payment of 50
Card balance is now -1970, lower than the original -2000 balance.