In the attached plot I show a comparison of some WEB sensors before (purple) and after (blue) that the auxiliary UPS (faulty in Aug. 27, due to a mouse) was put back in operation (September the 3rd).

It seems that something has changed on the "S" line: there are very wide sidebands that pop up (+/-7Hz from the 50Hz mains) not present before. These sidebands create a magnetic field that is sensed by all three magnetic sensors that are very close to the base of the tower.

I've also checked the old data, it seems this noise was not present before Sep. the 3rd.

Hard to say if it is a noise generated by the second UPS, or if it is due to a load on the "S" line. We should do a selective shutdown of both UPSs (once at a time) just to better understand what is happening.