I found that editing a UTF-8 file (says so correctly on the statusbar) results in a corrupted file when saved.
I've reported anomalies when viewing files before (see weird characters in plain text files), showing all kind of weird characters. You said it had to do with the RTF control you use and that the underlying data was not corrupted.
But now it is!! When I save such a file (usually when I've changed something) it sometimes gets truncated, because there's probably an end-of-file character in there somewhere, due to the RTF anomalies I suspect.
This is something which will have to be fixed, because you canot rely on E² saving the file correctly this way
I'll send you a pm with the file I'm having trouble with. Maybe you can make something out of it (I hope)

