This error is raised where there are duplicate Enhanced Rate Days added to the system. You can check for this by navigating to Enhanced Rate Days page which is found under Payments then Reference Data, within Admin/Intranet. Check through the ordered list of dates and remove any duplicates.
Adding duplicate dates is not possible as of version 21.1. This functionality was amended under the dev item ABA-49. These duplicates can cause problems with closing or updating service orders too.
Internal Solution
Check the Enhanced Rate Days as mentioned in the solution and advise the customer to remove any duplicates.
This can be tested locally if you have recent enough data, but the customer is able to check these entries on their end.