I own a Soundcraft Sprit Folio Lite since 1995. It worked fine for many
years, then it began distorting on the monitor output. So I had it
serviced.

When I took it back home and connected to its external power supply, it
showed a very loud hum (presumably AC), lighting LEDs up to -10dB.

I brought it back to the lab, but there it worked perfectly and sounded
great. They even changed the power supply connector with a full-metal
screw-lock one, but when again at home the mixer shows a very strong
hum, even with no cables connected (only the external power supply).

I have tried the mixer in many flats of my building, but the problem is
the same.

Ground problem? Bad AC power in the entire building? Is there any way
to discern what the problem is?