When I restarted my machine, and with it Qt Creator 4.3.0 (it was OK in 4.2.2), I reopened my previously two opened CMake projects. The second one contains an executable target (the first one is just a library).
When I right clicked on the second project name and chose Run, nothing happened. Nothing was printed to the console ("Application output"), it remained disabled-grey.
When I entered the Project configuration, I saw that the "Run" config name was changed from "project_name" to "project_name (disabled)" and the two field below it (command line arguments and working directory) were disabled, too. I could not delete the config, of course.
When I added a new one (with cloning), its fields also inherited the disabled attributes. However, after deleting the cloned one and adding again (it got into the drop-down box of the "Add" entry), its fields got enabled, although the command line arguments were deleted only the working directory was remembered.