S2_6 POTFILE.in refers to italy scenario by its old filename
S2_6 POTFILES.in was not properly updated when italy scenario filename was changed. That causes potfile generation to fail.
Later branches are not affected as there the filename did not change (there it does not include map size or version information)
As this makes, e.g., Windows installer build (both msys1 and msys2), which is the most feasible way to make any windows builds, to faiil, I'll push the patch as an build emergency fix.