Same issue here.
I created an account. It came with a default german iban because i was on holiday when creating it. Removed that account and created a new one with a durch iban as the default invoicing account.
Now when i try to use ideal i get the "no monetary account found for the alias" mentioning my initial german iban. If i restore that account ideal works again but now i have to always switch to the rother account in the app.
Default invoicing account should also set it as default for ideal. And removing an account that is the default for ideal should also change te default to an existing account.
As a workaround Niels you could try restoring deleted accounts from the archive and see if that helps.
Also described here: https://together.bunq.com/d/39420-change-the-default-ideal-account