Concerning the "nodata value in the input rasters set", I meant the "ignore_nodata" set (optional) in the focal st. window tool.
Regarding the "...05.tif" file, it showed no nodata, as you can see in the fig.
Therefore, according to what you wrote the nodata lack in the input file ("...05.tif") may be the problem, at this point. In order to get a smoother surface, how could I solve this issue?
I wanted a smoother surface to use it as input for the least cost path analysis, in order to avoid that the process could take into account small and medium surface variations/modern structures (such as a street or a roundabout) during the path estimation.