unable to open tree using unifrac.unweighted

what could be the reason why a tree does not open in the latest mothurversion but it did in 1.19?
Its the same file!
Do I need any support / additional files in my “mothurworkfolder” I might have forgotten?

Could you post the commands you are running? Did you look at this http://www.mothur.org/wiki/Frequently_asked_questions#Mothur_can.27t_find_my_input_files?