As of 4th of December, 2023, the issue resolved.
On the New Support Case (SP203500) form of the Acumatica Customer and Partner Portal, when clicking Submit button, the system returned the following error: “Error: Another process has updated the 'SACustomerSite' record. Your changes will be lost.”.
However, in the URL in the address bar, as shown in the below screenshot, the CaseCD has appeared indicating that the case was succesfully created.
Since the case in fact was created, to proceed working with it, access it using the My Support Cases (SP2040PL) form.
We are working on resolving the issue.