It was not a problem with fans, it is the bloody green-friendly UPSs : A momentary voltage drop (a couple of seconds) was enough to kill again nodes n0003 & n0004. Replace these UPSs with proper APC 700VA units ?

Replaced chassis fans on n0003, n0004, n0006. Removed side panel on n0009, will have to watch temperatures.

First impression is that temperatures are looking good. A graph will appear after a week-or-so …