Some more experiments:

Three sebsequent grinds, from a bit coarser than lately to very coarse made with tye Gaggia as it is way easier to do a couple of back to back shots than with the faemina and both produce the same sourness.

Result:

1 slow flow with nice looking stripes ( for these beans), taste sour with nice body if you ignore the acidity

2 normal flow nice striping, sour taste with ok body

3 fast flow, some schpritzers, sour taste lack of body

I am confused. You mentioned that your espresso has a volume of 25 ml and it is weighted. What is the weight of the espresso then?

Some people on this board experienced a change in perception of taste after taking some antibiotics. Are you on medication or have you change your medicine recently?

If it is not the beans, it is you, no offense. Please post a video of your bottomless extraction ("your" bottomless extraction lol ). That will help greatly.

EDITED: This site also supports vimeo. Try using that instead?

25 ml measured by weighing the result, 1 g of water is 1ml after all.
LOL no maybe I should be on medication soon if this keeps up, but please note; my GF and others would then have the same taste perversion..

i'm most happy to accept myself as the culprit, the thing that makes that theory difficult is that I'm using the same routine (how sloppy it might be, don't think so but still) as before and my results used to be quite good, both on the faemina and the GC.

Edit: coarser grind resulting in a quicker flow showing nice extraction using 16 g in and getting 35 ml out produced the same battery acid as before...

Well I think the reason has been found, I went to my barista of trust whose espresso is beyond suspicion and ordered a doppio From his Spirit...and it had the same metallic acidity as my own espresso...
So it must be our taste buds being off and most likey caused by quitting smoking a couple of weeks ago...

The doppio had the now familiar acid twang but with lots of body and depth, on one side I'm happy to probably have found the reason but on the other side I'm hoping this acidity is not going to stay.
All I've read so far about this taste perversion is people saying their taste sense improved and a few people reporting a bad taste/bitter taste...

As it turns out this friend of me who also experienced acidic espresso drastically cut back on his nicotine intake a few weeks ago...serendipidity does exist.

Can also add that I made up my mind to own a Kees vd Westen at some point in life...what a machine