Qt Creator crashes on startup, but it worked before

I installed the latest Qt version (5.5.1), and Qt creator worked fine, I could build programs with it. However, after I installed cmake, downloaded opencv from source, and built it, Qt creator no longer works. It silently crashes on startup, and if I start it from the console, it just says "Segmentation fault", nothing more. I didn't put cmake and opencv in the title of this question, because I'm not sure if they are really the culprits. The following happened after I successfully used Qt: - I tried to install a Radeon driver to use a second screen, by following https://wiki.debian.org/ATIProprietary , but it didn't work. (the installation was apparently successful, but my secondary video card is still not detected) - I installed cmake, together with its gui (called cmake-gui) - I downloaded the opencv source, and successfully built it, by specifying the 5.5.1 Qt folders as the location of Qt - During all of the above, one update was performed on the system. (apt-get update and apt-get upgrade) I'm suspecting cmake, because its gui uses Qt4, so it might have installed some stuff from Qt4 which might be causing conflicts - however, it seems that Qt4 was already installed with KDE, because I've seen it in the "software management". I tried deleting and installing Qt5.5.1 again, with no effect. I'm using KDE, and I cannot even uninstall Qt5.5.1 properly. It doesn't appear in the Software management - System settings, and when I right-click on its shortcut in the "start menu", I get the option to uninstall, but I'm greeted with The file could not be found in any installed package - so apparently my system doesn't seem to see Qt at all. (I can't even find any way to remove the shortcut, or even find where the executable is by right clicking, I had to locate the application manually - either there are serious UX issues, or I'm not experienced enough with the UI of KDE) I'm using debian 8 (jessie)