I understand that you can start the instance and connect to the WebPortal, but the username/password don’t work. Is that correct?
Did you also remove the hyphens ("-") from the MAC address? Maybe the instance has multiple network interfaces / MAC addresses?
the default password is set to the VM ID of the VM on the first launch of the KNIME Server. To recreate the password: obtain the VM ID of the VM from the Azure Portal console. When viewing the VM in the console, click on “JSON view” and copy the value in the “vmId” field.