No, I couldn't pinpoint it on one particular font. I remember I had just added some of the fonts
recommanded in this thread. After fixing the problem, I was able to transfer anew several TrueType fonts, including the URW fonts and some who are packaged with MS Windows.
Edit: After some tests, I'm thinking the problem is not with a particular font, but with a bug in the font managing software. I tried added more fonts, and when the height of the Font family window gets bigger than the screen height, it freezes or reboots when I try to change the font. I got to that stage when there were 16 font families in the window. So, to be on the safe side, it's better to transfer no more than 13 different font families in the memory of your reader, in addition to the 2 (Verdana and Courrier New) already present when you purchase the Cybook Gen3.