Libreoffice calc appears in launcher as a white icon

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http://askubuntu.com – When i try to open a .xls file sometimes appear a white icon on launcher instead of the green open office calc icon. Besides, when this problem happens, it doesnt allow me to open more than one file at a time and of course, the arrows next to the icon doesn't appear as well, What is happening? is there a way to solve this? (HowTos)