Thank you kekvitirae for trying to help.
I found the culprit - I have two MTW version (MTW-orig. and MTW-Mod) on my c: drive. Obviously the GnomEditor can not discriminate between the two; hence the conflicting changes when switching from Gnom to Notepad.
I think I should eliminate one version and try again.
I will try again and let you know later what the result was.