Error with event reg and open invoices iPart when OBO a certain user

When acting OBO a specific record and trying to either register for an event or view the open invoices ipart, I receive the "There was a problem loading this area." error.

This is the corresponding error from Event Viewer:

2022-04-12 10:25:41,002 [35] ERROR UKCLOUDSQL2:ML_Prod [(null)] - Error occured at Uri: https://HIDDEN.imiscloud.com/Asi.Scheduler_/api/PartyCommercePreferences/1171
System.NullReferenceException: Object reference not set to an instance of an object.
at Asi.Soa.V10.Commerce.Persistence.PartyCommercePreferencesLinqDefinition.<>c.<get_AvailableBillToParties>b__26_1(BillToPartiesLinqDefinition btp)
at System.Linq.EnumerableSorter2.ComputeKeys(TElement[] elements, Int32 count) at System.Linq.EnumerableSorter2.ComputeKeys(TElement[] elements, Int32 count)
at System.Linq.EnumerableSorter1.Sort(TElement[] elements, Int32 count) at System.Linq.OrderedEnumerable1.

If anyone could shed any light on what may be causing this, it would be much appreciated.

Thanks in advance.