How to tile a pattern in Illustrator?

I have joined two same sized rounded rectangles creating 90 degrees. Then I made 45 degrees guide and copied figure so they both can be on the same guide.

Then using a blend tool I evenly distributed figures every 9mm.The result I copied and pasted every 24 mm to get this piece of pattern:

Now I want to crop it and save as a pattern. But before that I did Object > Expand ang grouped it to prepare it for cropping. As on the previous stage it wouldn't crop at all. I'm not sure how to do it right so I created a square and placed it in the centre of the group.

After cropping I've got some weird lines on the shapes. Also my pattern does not tile smoothly. Here is the result:

Why do I have these thin lines on rounded rectangles? And more importantly, how can I make this pattern tile smoothly?