Thank you, especially davidfor an kovidgoyal, for the fast reaction!
Reporting a bug is sometimes discouraging, as some programmers may be haughty, and you get also answers from people who pretend to know more than you while they did not understand the question...
As I have been a hobby programmer for a rather long time (but rather long ago), I think I'll have a glance at the code, because the sort of error found by David is exactly the same as some I found in the last beta of WinFF some months ago
Thank you, Kovid, for the great software!