Small guess but due to Intel buying up Itseez, the repo names were changed. This means the for now the repostories are changed to
- https://github.com/OpenCV/opencv.git
- https://github.com/OpenCV/opencv_contrib.git
- https://github.com/OpenCV/opencv_extra.git
So if any issues turn up locally in your git configuration, then adapt your remote to the new address :)