19/02/2017: rewrited issue, screenshots added, and added information about the .pk3 involved (0pics.pk3).

The box lines of the inventory for soldiers (right hand, left hand, backpack, etc.) are not visible when .pk3 files only are used to run the game (Cf. http://ufoai.org/forum/index.php/topic,9066.msg65352.html#new). In the forum thread, I mentioned the XVI button, but I am probably wrong because I started a campaign without unzipping the .pk3, and unzipped them only a few days after, while the campaign was already a little bit advanced.

Perhaps other items of the game are concerned with this issue; couldn't check it. Only windows version was tested; other platforms were not, but might be affected

Workaround:
# install the most recent v2.6 build
# go to the folder where the game is installed
# unzip 0pics.pk3 (example with 7-zip: "Extract here" option)
# move this .pk3 to another folder (not in the UFO:AI folder)
Note: this workaround was first done by unzipping all the .pk3, but unzipping 0pics.pk3 is enough