I haven’t used the Dell configuration key for a while. Today I accidentally touched it, and an error popped up:
Error found! Submit this issue to QuickSet owner!
Based on this post, this issue is caused by Intel® PROSet/Wireless Software for Bluetooth® technology with version above 3. So I uninstalled the Intel® PROSet/Wireless Software for Bluetooth® technology 3.0.1304, and installed version 2.6.1212 back (which can be downloaded here http://www.intel.com/support/wireless/wtech/proset-ws/sb/CS-034060.htm). It is all good now.