maybe a case for @WLBI :)
A cese for me? ;-)
Well, in my BQ M10 Tab, the IMEI was also gone and I need to enter the WiFI password after reboot, when I want to connect again to the same WIFI.
The problem was, the NVRAM was broken. I contact BQ and I can send my old device to them. They replaced it with a new one. It was was warranty case after 1 1/2 years.
Edit: The IMEI was not gone, it was:: 0123456789ABCDEF