iPerf3 give this error when it starts as a service? It’s not related to the iPerf version, and it’s a hardware/network problem!

Most probably, it’s a congestion problem with your VPS, and here are the steps I made using tcpdump.

#ethtool -S ens33 | grep -i drop
drv dropped tx total: 0
drv dropped tx total: 0
drv dropped rx total: 0
drv dropped rx total: 0

and

#netstat -p 8000 tcp -s | grep error (port 8000 on my server)
0 packet receive errors
0 receive buffer errors
0 send buffer errors

If nothing there, continue with tcpdump

#tcpdump -i $interface -npvs0 icmp or ‘tcp[tcpflags] & (tcp-rst) != 0’ | grep incorrect
…
dropped privs to tcpdump
tcpdump: listening on ens33, link-type EN10MB (Ethernet), capture size 262144 bytes
188.208.*.114.irdmi > 188.208.*.45.56176: Flags [R.], cksum 0x7b80 (incorrect -> 0xe67a), seq 3620189790, ack 1510148885, win 24576, length 0

….

In my case, it was a setting on ESXi. 🙂