Issues with Peoplesoft AR when using Post Now option after applying a payment.

 My customer is having issues posting AR payment as using the Post Now option at Peoplesoft AR.  Their is only one user registering and posting payments but the system returns the following message " Group contains a BU/Customer currently being processed by AR Update. Please try again later (6000, 110).  This group contains a Pending Item for a Business Unit/Customer that is also in a group processed by another Post Now or Batch run of AR Receivable Update (AR Update).  Please try again later after the other run of AR Receivable Update completes successfully.
This situation is occurring with one user and we want to know how we can avoid this situation when customer is registering payment and using the Post Now option.  Their could be instances in which we could have more than one user recording payments.