A network engineer configures an IP SLA UDP jitter operation on R1 to measure latency and jitter to R2 across a WAN link. The operation is scheduled and active, but 'show ip sla statistics' on R1 shows a 'Return Code' of 'No connection' and no jitter or latency data is collected. ICMP echo operations to the same R2 address succeed normally. What is the most likely cause?