This turned out to be an encoding issue. I had the following line in my web.config file:
<globalization enableClientBasedCulture="true" culture="nl-NL" uiCulture="nl-NL" requestEncoding="ISO-8859-15" responseEncoding="ISO-8859-15" fileEncoding="iso-8859-15" />
Changing that to:
<globalization enableClientBasedCulture="true" culture="nl-NL" uiCulture="nl-NL" />
solved the issue.
At this point I'm not really sure what effect the removal of those encoding properties will have elsewhere in my application, but it did solve the CuteEditor problem.
Regards, Marja