Sales Order Not saved with error Failed to load resource

Hi guys

We have upgraded to
ERPNext: v13.0.0-beta.4 (HEAD)
Frappe Framework: v13.0.0-beta.5 (HEAD)

Now when I create a new sales order and want to save it, it is not saved without showing any message. From console I can see following error:

Failed to load resource: the server responded with a status of 500 (INTERNAL SERVER ERROR) api/method/

Can anyone help?