Hi there!
Something goed wrong from the Forms Portal, trying to enter the new Group Forms.
I found a workaround though, that might help fixing the bug:
1. I open the Forms portal
2. I click on this "SilentSignIn" URL (see below)
3. I go back to the Forms Portal tab and press the GroupForms button: now it works.
4. Change from "Recent Forms" to your desired Team or Group.
5. Now you can create a Form or Quiz.
Don't ask me why...
https://login.microsoftonline.com/common/oauth2/authorize?response_mode=form_post&response_type=id_token+code&scope=openid&mkt=en-US&msafed=0&nonce=c7ff9601-e096-4eba-bce7-575fdacb181a.636639104464797920&state=https:%2f%2fforms.office.com%2fPages%2fSilentSignInComplete.aspx&client_id=c9a559d2-7aab-4f13-a6ed-e7e9c52aec87&redirect_uri=https:%2f%2fforms.office.com%2fauth%2fsignin