Sim cards keep getting locked without any reason

ithuriel

Yesterday I got the phone, it's great, but I've got an issue with my Sim cards. After a while the phone randomly decides that the Sim cards are locked and when I try to use it asks me to unlock the Sim cards by entering the pin code. The device didn't restart, I didn't removed the Sim tray, so these aren't the issue. The problem started after updating my phone today. There weren't anny issue with it yesterday, but today it already happened 3 times after the update. What could be the issue? Could anyone help me out?

Thanks in advance!

P.S.
Is there an easy way to downgrade my phone to the original version it came with? Because my issues started after updating the phone.