If you are on any past or future period (not the current month), and you try to create a new transaction on a narrow screen (via the side panel), the date field is automatically filled in to the first of that month.

If you don't edit the date, it will fail with an "Error creating transaction".

If you click the date field and manually set the date (even if you choose the exact same date that was already there), it will resolve the issue and you'll be able to successfully create the transaction.