Found it! There was a lirc daemon running in background, needed for my previous remote. It ate all input done by flirc, don't know why. While having the text editor focused in foreground and mythfrontend in background all input was directed to mythfrontend and nothing to the editor. After stopping lirc, flirc input was put through to editor (all programmed keys) and to mythfrontend too.
I've installed the configuration software in Windows and configured flirc there. In Windows the sent keys are recognized, like an "m" or numbers. I can see them in a text editor. I've put then flirc into a linux box (Mythbuntu) and watched the commands here. I've installed no flirc software here imaging that flirc works as a simple keyboard here and doesn't need any additional driver or software. Cursor keys for instance are recognized here but no letters like "m" or even numbers. What can be the reason for these differences?