I installed CyanogenMod 10 Nightlies on my Galaxy S1, coming from CyanogenMod 9.
On every reboot I get the message:
Warning! A problem was detected with your device. Your device IMEI number is invalid. And invalid IMEI number could cause network issues including the inability to call emergency numbers.
Sometimes I have network issues and my handy just won't connect to the Internet(even though it shows E/H Connection)
I once made a backup of my
/efs/ partition, I already restored it but the message still appears.