Hi kjell.ek,
We have fixed the updatepanel issue in the latest build. I suggest you download the latest build and try again.
Latest build download url: http://richtexteditor.com/download/richtexteditor.rar
Please replace the dll file and the richtexteditor folder.
If you still get the same issue with the latest build, please create an example page which can reproduce this issue and post here. We will check it and get back to you as soon as possible.
Regards,
Ken