I removed stuff that doesn't affect the bug. In particular the tree that has caused problems before, two layers water and reflective stuff all over the place. This allows faster rendering = faster testing, removes some potential sources of the problem and hopefully makes finding the bug easier.
The quality of the clouds is bad because I only left the Quick Render node in with no anti aliasing and detail at 0.5 which pretty much destroys the benefit you get from the high number of samples.
Forgot to mention: I use 4 threads and I did let the render complete - the former might be the reason why you don't see the problem. Although I think I do see lines in your render.. but that's hard to tell with those compression artifacts.
Here's what I get (as a png, not lossless, but preserves the edges which jpeg is pretty bad at):
[attach=0]
@TU Now that you mention it. I get similar problems with grass if the displacement moves parts of it across tile borders. That's a problem with displacement and tiles (there's a thread about this somewhere around here but I couldn't find it).. I don't know if it's the same problem as yours.