Yes, previously downloaded with no conversion an worked in Calibre 0.7x versions.
However, just to complicate matters further, the books work fine in the ereader in Calibre and in mobireader.
I tried re-formatting my cybook card and re-installing that in case something funny had happened with that, but no difference. Some books work ok, others re-boot at last few pages.
Well, I have just found a solution. Some .prc books needs to be run through Calibre convert to mobi format. Then delete the prc version from Calibre library. Then send it to Cybook. Then it works! Weird!
But never had this problem before v0.8