I have seen a few questions on Technet Forums recently which were related to Notes section in Exchange 2010 OWA.
The symptom is that a user is not able to create any notes after selecting Notes folder.
I just learnt that this Notes folder is left behind and will be unsupported in OWA. So, currently there is no way that you can fix this problem and get notes created using OWA. You must use Outlook for doing so.