When you create an invoice in onCourse, the general ledger is not immediately credited with the income from training delivery. Instead, a liability is created called "prepaid fees". onCourse then journals that liability to income as the class delivery takes place. Some onCourse users don't use our general ledger or worry about pre-paid fees. In that, case this issue will not affect you.
A defect was discovered that has existed since onCourse 4.0. The issue is triggered when a student had only partially paid for one invoice, then enrolled in another class, creating a another invoice. The remaining fee liability component from the first unpaid invoice (or invoices) was reset to be the full balance of the original income, resulting in a potential over allocation of income from the liability account for that invoice. On upgrade to 6.0.1, the over allocated invoice's income will be debited against the appropriate income account and credited to the fee liability account to bring all account balance back to the correct position. You will see these paired transaction lines posted on the date the upgrade takes place.
This is a once off adjustment which will bring your general ledger back into the correct position. For most users this means you will see a decrease in your income that corresponds with a decrease in your liability. This fix also prevents this error from occurring on any future invoices.
If you are using the new parent/child relationships for online enrolments, this fix will allow those relationships to flow back from the website to your onCourse system. All historic data will automatically be synchronised.