[Riva3000]

I kept thinking about it and I think I found the problem and solution.

I disected and compared 2 ePub files. both with same diacritics. One workin on iRex, one not.
I found that the broken one, made by Calibre..
in file stylesheet.css
sets for every style definition a: font-family: "Times New Roman";
The working ePub has most files similar but the stylesheet.css doesnt set font-family at all.

So I tryed to edit out the tags and the edited ePub now read right on the iRex.

I'll have to try it with other files/ePubs to be sure its the whole and only cause of the problem.

I don't know much about ePub standard so I cant tell if its a bug in Calibre, or bug in iRex FW, or just a different format implementation.
If somebody can tell, please eventually report a Calibre bug.