After shutdown and restart of my Linux Slackware 12 server everything came up the way it should. The reason to shut down the server whas to re-locate the machine.

But, at random intervals, eth1 goes down. Syslog and Messages doesn't show anything about it. I know I haven't given eth1 order to go down.
Configuration is as follows:
eth0 - 192.168.168.x (local network) -> switch
eth1 - 10.0.0.1 -> adsl modem 10.0.0.138 -> internet

The box has been running for 2 years without any problem. Didn't make any changes to the configuration so I have no clue why eth1 goes down. After bringing it up again it goes perfectly for ... well some time.

What is going on?

It seems openVPN is to blaim for this problem. Some need to make openVPN connections with this server. But when doing so, the connection failed and that is the moment eth1 goes down. After re-installing the openVPN server eth1 is still up (for about 13 hours now), while openVPN connections work again.

Still don't know what was wrong but it seems solved, eth1 is still up and the openVPN server is running fine.