Zoran,,, I have attached an assembly file of your file and your file modified so you can see the (very slight) differences. I deleted your symmetrical relation in Sketch2 and and made a parallel relation. This works but does change your geometry slightly. I this doesn't work for you I have another method but I think you probably want planar surfaces. Two bends instead of 4...
Edit:.... I attached another example (different method) which uses your sketch2 dims and relations and results in non-planar sides. Flattens without issues.
This is the one with FIT on the end of the file name.
Your problem is with Sketch2. The dimensions you are using result in the angled sides NOT being parallel to the Sketch1 lines.
The fix is to change the sketch geometry of Sketch2. DELETE ALL LENGTH DIMENSIONS. Make the angled lines parallel to Sketch1 with a distance of 5 mm. Make the horizontal line a distance of 5 mm from Sketch1.
SolidWorks now has planar surfaces that it can work with to flatten the lofted bend..
Thank You guys! You saved my time!