Network Problem Linux

I am currently working with Linux (4.9.246) on VC707 board with Leon5. I am using the template of the GRLIB IP 2021.2b4267 library, which I installed with vivado 2018.1. Linux is running normally so far, except for a problem with the network. With a normal size of 56 bytes the packets do not arrive:

ping -s 8
PING ( 8 data bytes
--- ping statistics ---
5 packets transmitted, 5 packets received, 0% packet loss
round-trip min/avg/max = 1.051/1.316/2.034 ms

ping -s 30
PING ( 30 data bytes
8 packets transmitted, 4 packets received, 50% packet loss
round-trip min/avg/max = 1.054/1.395/2.075 ms

PING ( 56 data bytes
--- ping statistics ---
21 packets transmitted, 1 packets received, 95% packet loss
round-trip min/avg/max = 1.376/1.376/1.376 ms

I used the settings from the Linux User Guide. (“Aeroflex Gaisler GRETH Ethernet MAC support” (GRETH)").
To exclude that it is due to the kernel version, I also tested the image of version 5.10, but the same problem appears. Can you help me to find out what the problem could be or how to fix it?