Low Z mode is great for stray leakage and confirming capacitive ghost voltages, but will trip the RCD every time, unless DUT is isolated from the mains.
FWIW: some meter brands need to cool down (up to one hour) after using Low Z mode on mains voltages,
as resistance readings in Ohms mode may not be stable or accurate with the user believing he/she has a lemon meter
I've played with UPS units in the past and noticed on some that the input Live and Neutral can be wrongly wired by the manufacturer,
especially "international market wiring colour scheme" snafus > black and white vs brown and blue vs red and black
vs drag revue pink and purple
AND I've observed Live and Neutral flipped/reversed on the UPS output during a power outage as referenced to the input sine wave
Some can be wired properly so that input and output POLARITY is correct, with some expected voltage/current phase shift difference expected
By saying 'Some can wired be properly' I mean if the rewire does not affect the battery charging circuit and battery if there are common connections.
If the UPS output runs through an isolation transformer, like some APC SmartUPS and similar Compaq/HP IIRC, those are easy to correct -if- they are out of wack.
Yes, some are, most not, luck of the draw.
Still, their transformer output is isolated (many times via two 120v legs plus CT feeding 240v to L and N output) with earth/ground still connected at the wall socket,
so the connected load/equipment won't care
All my blab above assumes correctly wired wall socket/s, IEC kettle cord leads, decent power strip boards,
with no moisture or short causing dirt/metal particle ingress anywhere.
and non flipped or poorly repaired DIY extension cords (classic widow makers)
@ hgg: we may never know if the meter fuse blew because of what happened somehow with the UPS, or from a previous test, a severe meter drop/vibration,
or the fuse was a rare dud,
or someone borrowed the meter and zapped it with voltage on the current setting, whatever...
The good news is that it got you pumped to find out what the go was, and corrected a far bigger problem/issue
...than worrying about the euros you will blow on another fuse
YES, they are WAY-Y-Y overpriced for what they are,
another necessary credit card
rape item
but still a bit cheaper than buying a replacement meter and or blowing money at a hospital/funeral
...and the sellers know it

The sneaky cheaper knockoff fuse floggers aren't doing the electronics community any favours either
**** them