I've watched the Nick True video that came highly recommended in a bunch of Reddit results (e.g. here), but I still don't understand what's going on.
- Have all funds assigned, so that "Ready To Assign" is at zero.
- Approve a(n automatically imported) transfer from my checking account to my credit card account (see screenshot). Note that it is registered as an inter-account Payment and not as a regular expense.
- Budget view shows an equivalent amount of money "Ready To Assign".
Confirmed, all categories have non-negative amounts in the "Available" column.
I'd recommend reaching out to YNAB support. They can even view your budget (if you give them permission) and determine if it is a bug or something is configured wrong.