From my "About Qt Creator":
Qt Creator 4.0.0
Based on Qt 5.6.0 (Clang 7.0 (Apple), 64 bit)
Built on May 10 2016 01:11:54
From revision 605ea627cc

It was working fine before. It might be due to an Xcode update (or not, I'm not sure).

Most of the time, The debug process ended with nothing happening. Log in "Application Output" is:
"Debugging starts
Debugging has finished"
And the application has not been executed at all.

Occasionally I got an error message (in a popup window) stating:
"An error occurred when attempting to write to the LLDB process. For example, the process may not be running, or it may have closed its input channel."

In Preferences > Build & Run > Kits, "Desktop (default)" is the chosen kit, and I tried both /usr/bin/lldb and /Application/Xcode.app/Content/Developer/usr/bin/lldb for Debugger option. But both ended not actually running the debug session.