ok, I've gotten it working.
I believed I was running the latest version because my version number was 6.2.0.0
However, whilst attempting to reproduce the error in a sandbox app I downloaded the latest version from the cutesoft site.
Both versions of the dll are marked as version 6.2.0.0 but the version I just downloaded works whilst the one I already had doesn't.
Looking closer, my CuteEditor.dll has a build date of 2008-10-15 whilst the one I just downloaded has a date of 2008-12-10.
This issue might well have been avoided if Cutesoft had issued an email that prompted customers to download a bug-fixed version.
Additionally, you really should increment the build number when releasing altered software.