Lines slightly elevated, can't flatten

I try to make a block, the insertion point is the corner inside the red circle.
The z coordinate shows something like 0.0000000068
The smallest precision in the UNITS control is less: 0.00000000, so it doesn't help.
Thus the ID command shoes only 0.00000000
FLATTEN causes troubles, it destroyes some of the shapes that have the same characteristics.
CHANGE-->Elev shows O.K.: 0.00000000
I read about a round off trick, where can i find it?

You can use the brute force method, described here

I tested it on your drawing and it seemed to work.