Filling out a form: unlicensed user is shown error message

We have used Microsoft Forms succesfully for unlicensed users (parents of our students are part of Azure AD, but are not licensed for Office 365). They are on a separate domain, which is enabled for the tenant.

However, suddenly these users are shown an error message when filling out a form:

Which says something like 'The account is not enabled for MS Forms'.

I can't find a setting for fixing this. The only setting is enabling forms to enable for external users, that is enabled. Is there another setting I'm missing, besides assigning a license for these users (which should not be necessary for filling out forms, according to MS: "Removing the license from a user will prevent the user from creating forms in Microsoft Forms, but they can still respond to forms created by others".