Kodi freezes after some minutes

I have been running Libreelec on a Raspberry Pi 3 for quite some time without any problems. The videos are played from a NFS share on another Raspberry Pi.

But since a few days every video freezes completely after a few minutes and Kodi doesn't react to any input anymore. But with ssh I can restart Kodi without rebooting the whole RPi.

The server's log is completely unobtrusive - no hint at all.

The system log (journalctl) of the Kodi-Pi shows nothing conspicuous.

Only in the Kodi-Log the last message is "CRenderManager::WaitForBuffer - timeout waiting for buffer".

I have tried the following so far:

- Replaced the Pi by another one.

- LibreElec completely reinstalled.

- changed from Libreelec 9.0.2 to 9.1.0001.

- Samba used instead of NFS.

- disabled Automatic hard disk sleep on the server.

If anyone had any tips where to look for the cause I would be extremely grateful.

Which additional information would be helpful?

I already had a LAN problem in mind, so I just have a ping from libreelec-pi to the server for over an hour. Not a single packet was lost.

To the setup: Both are directly connected to the same switch by cable. So not even a router in between.