The subscription has NULL next billing date (NextBillDate), as a result some of the operations (e.g. attempt to switch service plan) fail with exception:
operation with NULL value
It is possible to empty "Next Billing Date" for subscription by editing it from PCP. The issue is considered as a bug with id #PBA-68849 (""Next billing date" should not be Empty").
Set the correct next billing date for the subscription and repeat the previous operation.