Mini sudden problem in epub files

i bought my kobo mini 2 weeks ago. it was great. today i sideloaded some epubs from calibre and when i tried to go to a chapter in one of those books whole reading section went black. this happened for all previous and recent side loaded
epubs but not with thebooks i bought from kobo.:blink:

has this happened to anyone before? any ideas what i should do?:help:

(sorry for poor english:o)