Stock debian image - rock64 - gigabit

[rhille]

When using my Rock64 with Gigabit link, network connections are unstable (I even cannot get the dmesg output via ssh connection), no problem when using 100Mbit.

dmesg doesn't indicate any problem (retrieved after set port to 100 MBit)
eth0: Link is Up - 1Gbps/Full - flow control rx/tx
eth0: Link is Up - 100Mbps/Full - flow control rx/tx

Did a sudo apt-get update/upgrade which updated a couple of things but issue persist

Current setup:
Rock64 
4GB 
official debian image on SD card
emmc plugged in but not used yet.
Router/Switch: Fritzbox 6360

When using the same FritzBox port (configured as Gigabit port) with my laptop, no issue is seen.

Any ideas?

Thank you
Reinhard