Leaking CO2 regulator

[TheCount]

Hello!
So I've had a leaking system for a long time now (since i bought my CO2 equipment actually), preventing me from using kegs. I've emptied six sodastream bottles trying to find the leaks but this time i tested to screw the thing on the back of the regulator. The thing is, since i screwed it back it started to leak from it. It seems like it isn't completely shut but I can't see any springs and it's rock solid.

So now to my question. What even is this thing to begin with? How can I fix this (new and obvious) leak?

My regulator looks similar to this (see the link below) and it's the circular thing on the opposite side from the nipple I'm referring to.
https://www.amazon.co.uk/Charger-St...keywords=co2+regulator&qid=1623611264&sr=8-15

 

[The-Engineer-That-Brews]

I’m not familiar with that kind of regulator, but from the photo I’d say you’re not intended to adjust or try to remove that part (opposite the nipple). If it’s leaking from there I’d return it - or if that’s not an option, fill up the gap round it with epoxy resin.