CMS HCAL VTRx-induced communication loss and mitigation

<!--HTML-->The Compact Muon Solenoid (CMS) Phase 1 Hadron Calorimeter Upgrade (HCAL) saw the first large-scale use of VTRx modules, optical transceivers designed to be radiation and magnetic field tolerant. During Run II of the Large Hadron Collider, the CMS HCAL experienced a short period of communication loss that revealed a manufacturing weakness in the VTRx affecting nearly half of the communication links. The CMS HCAL Team provided the first observation of this phenomenon, revealed its temperature dependence, and pioneered the mitigation tactic adopted by other LHC experiments. We will present these aspects of the larger VTRx investigation.