Greetings to all!
In good old times I've render x264 to 12k and 25k just setting the bitrate value at the Rendering options window. And the result was fine. Some time ago I've updated my Linux distro and started to use new KDEnlive with its new flexible system of rendering profiles. But even with crf=21 or 18 I get very pixelated video.
Here is the profile with that I render: "f=mp4 vcodec=libx264 acodec=aac ab=384k crf=21 g=25 properties=x264-medium"
I've tried other g=, bigger "g" gives me bigger pixelating. Maybe I'm wrong with the term "pixelating". The image becomes dirty.

"g" is libavcodec (FFmpeg) parameter for "group of pictures", which is the maximum number of P o B frames (partial, image differences) between 2 I frames (full image). Normally it doesn't affect "pixellation" in constant quality encoding! (using crf parameter).
Don't you have "use proxies" checkbox activated, which takes the low resolution proxies?