Synology NAS is configured with community bluejay_monitor (→ snmp.yml auth 'bluejay_v2'), not public. public_v2 was returning HTTP 500 from snmp-exporter for this target. Verified bluejay_v2 returns metrics. Keeps printer (10.0.58.107) on public_v2 — Epson ET-3750 uses community "public" as documented in its SNMP settings.