I'm not familiar with the exact regulatory text for fintech, but it's highly likely that there is a difference between suspended and closed accounts.
While it may be illegal to close an account without providing proof, it seems possible to soft-block or suspend an account for 'security reasons' without disclosing whether it's related to fraud prevention or other issues.
It's not illegal. Even though having access to banking is absolutely essential to every imaginable business, bank accounts are still treated as a voluntary business relationship by regulators. Some protections exist in the EU for individuals, but for businesses, it is entirely at the discretion of the bank to open and maintain a relationship.
While it may be illegal to close an account without providing proof, it seems possible to soft-block or suspend an account for 'security reasons' without disclosing whether it's related to fraud prevention or other issues.