Says I underpaid Credit Card but it's zero?
Feb 2020 I have a 0.00 to be budgeted, everything is green. My American Express balance is 0.00 paid for by my budget. I move to March and I suddenly have a -169.55 budget. I look around and flip back to Feb, in the weird 'Credit Card Payments' section is a -169.55 balance on my American Express....but this is incorrect. My American Express balance is 0.00, I paid for everything with money from the budget, where does YNAB get this information?
The AMEX category under the Credit Card payments group is how much of your cash you have reserved to pay the bill. It is not your account balance (which is in the account area on the left). Ideally, they would match (i.e., for a paid-in-full user), but not everyone is in that situation.
This is exactly like spending $150 on Groceries when there is only $100 Available in the Grocery category. YNAB warns you (in red) that you implicitly stole $50 from some other category and gave it to the grocery store.
In your case, you very likely did not reserve funds for the starting balance (although it might have been credit overspending in a previous month). YNAB only reserves funds for budgeted purchases (i.e., against a category with sufficient funds Available). There is nothing stopping you from sending the CC more than is in the category, but understand those additional dollars are earmarked for other purposes in the budget. Since they're now gone, YNAB warns you of this so you can reduce one or more other categories.
The follow-up to this is that YNAB will not let this "missing money" situation persist past that month. It will make up that difference from the following month's To Be Budgeted, with the expectation you will budget less elsewhere when you do get more money.
TLDR: Best practice is to check category balances before every outflow and arrange for enough money to be in the category in advance. Failing that, respond to YNAB's warnings to cover overspending after the fact in the same month. A paid-in-full credit card should have its Payment category cover the entire working account balance.
Thanks for the explanation, it's still weird how it got there, I input all my bills/spending every Friday, and every Friday I make sure all Categories are green, which means I've covered my spending with my budget..oh well...I'll just trust YNAB, I've been using it since v4 and it's never steered me wrong. I musta messed up someplace in a previous month :)