Same here… Exact same software version for Sonarr and Mono as OP.
Sonarr version (exact version) : 3.0.6.1342
Mono version (Running on Syno) : 5.20.1.34-16
OS : Synology NAS ( Version: 6.2.4-25556 Update 2 )
Logs are exactly the same as OP, this also happens while trying to connect to NZB.su