well so far the problems seem to have been solved ... somehow ...
We did a reboot of the lfs.net server after which it did not come back online.
After having a closer look it became apparent that there is a physical network connection issue (either the network interface / cable or the switch). Chances are this is what cause the master server to behave irradically earlier on and a reboot surfaced the problem even more.
We're going to have to check which network component is faulty and replace it.
For now everything seems back to normal.