Thanks a lot to everyone for all your thoughts and suggestions. I tried different approaches, without being able to completely suppress this pukey/minerally taste but I still could limit it.
Here are the results of my experimentations.
1) Added some gypsum, some sodium and more calcium chloride to reach this profile:
Ca+2 Mg+2 Na+ Cl- SO4-2 HCO
134.3 11.5 24.9 199.2 149.6 0.000
Comment: still pukey/minerally but like on a higher note. Doesn’t really improve the beer.
2) Added some gypsum, a bit of sodium and a tiny bit more calcium chloride to reach this profile:
Ca+2 Mg+2 Na+ Cl- SO4-2 HCO
98.4 11.5 9.8 135.7 118.0 0.000
Comments: the higher sulfate does “mute” the hops a bit and increases the bitterness perception but it also covers/reduces the pukey/minerally taste. It’s still isn’t great but it’s a lot better.
My current thoughts: as I said, the water could’ve been infected and I still doubt that I’ll be able to fix this issue. I’ll only know for sure when I’ve done it on an entire keg (had problems mixing the brewing salts really well in 5 minutes in a glass of carbonated beer, maybe it will dissolve better over a few days in the keg, plus it wasn’t all that accurate since I was working with very tiny amounts with an only acceptable precision scale). But what if there was a kind of optimum (for my taste, for this particular beer) around 80-120 sulfate and 100-140 chloride? I’ll keep experimenting in this range and then finally decide myself to do it on an entire keg. I’ll keep you posted.