Ok, yet an other bug.
I had a messy ePub. (7Mg is too big i think
).
Trouble is, the opus tried to generate the thumbnail, and fail. The folder where the ePub was became totally inaccessible, as the opus showed the loading screen for a while, then when back to previous folder.
Freezing while reading still say first on the list of annoying bugs. My opus seam to have calmed down though.