@giukas I don't know :(. All I did was that I used some clean alcohol to wipe the SIM card and regarding the slot I've just looked inside if there is something visible that would cause the issue and haven't seen anything. So I just blowed some air there ... and it still wasn't ok since the contact was lost later again so what resolved the issue for me was to push the card into the slot thoroughly ... maybe I haven't had it fully inserted before ... there could have been some dust there that have prevented me to place the card correctly.
Not sure ... but be careful and try not to destroy the slot it if isn't yet :). Good luck.
PS. It could also be a SIM card issue. You may want to try another one to see if it works or try this one in a different phone.