Timo, thanks for your swift reply. Yes, I had everything about spelling switched off in Windows settings, including the setting "Spelfouten markeren" (highlighting errors). But still always the red underlining was there.
What I have done now, is turn the highlighting on in stead of off, do some typing, turn it off again, then exit and restart Diarium, and wow... finally it seems to work. I'm a bit anxious that the apparent bug would come back, but for now it works fine. No red wiggly lines anymore. It is such a world of difference, I hope it stays this way and I am really relieved. I will like Diarium so much more. Thanks for your help!