FOSS gamedev and creative worlds

Well I have installed minetest recently, in linux mint 15, runs nice, but when I craft to make stairs or slabs the output is a color square, with the color random I guess, because sometimes is black, yellow...and when I choose that square appears something like a rectangul that flashes the color, and when I click the left button dissapear. Anyone knows what is the problem and hpow to fixe?.

ps: Sorry for my english, is not my mother language.

Try turning off "preload item visuals" in the settings tab.