Cannot add Outlook account to Mail

My mum's Outlook account was locked due to too many incorrect password attempts, but after that was sorted and she could sign in at outlook.live.com, the Outlook account on her 9th Gen iPad still wouldn't accept the new password. The most common advice was to delete the account and then set it up again. I've deleted the account, but I can't put it back. In Settings > Apps > Mail > Accounts > Add Account, I click on Outlook.com and the Microsoft sign-in page appears, but entering her email address > Next > password > Sign In just closes the sign-in screen and takes me back to the Add Account page. Does anyone have any ideas as to why this is happening?