When downgrading the pricing plan to PROFESSIONAL, it was detected that the SSO (Single Sign-On) via Microsoft Entra ID feature is active, which is not available in this plan. Therefore, this feature needs to be deactivated.
The SSO feature can be disabled by an Administrator in Settings » SSO. Toggle the slider and save the changes by clicking the green Save button. After that, it will be possible to continue the switch to the PROFESSIONAL pricing plan.