Post over on the cactus and succulent forum, not the gallery. You have a better chance of getting an answer there, however the picture is not that good.

Your cactus needs a much smaller pot and faster draining potting mix. Also there is too much soil packed up around the base of the cactus, it needs to be moved back a bit. Also need more light if that is its permanent place. That screen prevents the cactus from getting adequate light.

Hey, thanks for the info, I will post it over at cactus and succulent forum later :)

You need to repot it in a smaller pot and more free draining mix. The mix you have is too peaty and will retain water causing rotting.