After upgrading to ubuntu 15.04 my Cairo Dock panel started to flicker making almost impossible to click on any icon in it. Reinstalling it didn't make any change.

This is how I've managed to fix the annoying problem.

First it must be noted that the problem only appears on laptops/desktops with an ATI video card. This is because ATI video cards (still) don't support GpenGL v2.

So if you run Cairo Dock for the first time after installing it shows up an window in which you can select the backend: OpenGL or Cairo. Check the Cairo option (if you have an ATI video card).

If, for some reason, this window doesn't appear (and this was my case) you have two options:

1. You have to start Cairo Dock from terminal running the comand:

   $ cairo-dock -c

   The unpleasant thing with this fix is you cannot use the Cairo Dock icon to start it and you have to run this command each time you want to get running the Cairo Dock.
2. The second option - and the more convenient - is to edit the .cairo-dock file in the Home > .config folder: open your Home folder, check the option Show hidden files in your file manager (Nautilus) and in your home folder will be shown all hidden directories. Navigate to the .config folder inside it enter the cairo-dock folder and there is the .cairo-dock file, open it in your favourite editor and change the line which reads:

   default backend=opengl

   to

   default backend=cairo

   save it and that's all! All you have to do is to restart Cairo-Dock and it'll work (again) like a charm. 

Happy (ro)coding!