"Fundamentally, there are systematic issues that have been discovered in the software that loads the [...] tables that [produce] the [affected] data. An extensive analysis has been underway to fully identify the root problems and provide design corrections. The logic involved is highly complex and requires considerable time to properly analyze. Unfortunately, we don't currently have clearly stated business rules for the associated data.
"Where we stand now is that the business rules which we have implemented are being reverse engineered as much as possible out of the current highly complex code. Following completion of this reverse engineering, discussions will be needed with the billing vendor to verify (and correct as appropriate) the rules obtained. Then corrections and adjustments will be made to the code. That said, we are weeks and perhaps months from having this addressed."
Tip: Don't fire everyone. Or, if you're going to do it anyway, make dang sure everything is fully documented.