1. If you upgrade from version 5.x, delete the following file:
 

CuteSoft_Client/CuteEditor/web.config 

2. If not, try add the following entry to your web.config file:
 

<add key="CuteEditorEnableGZip" value="False" /> 

Keep me posted

Another solution: download version 6.1:

http://cutesoft.net/downloads/default.aspx

Demo:

http://cutesoft.net/example/general.aspx

asp.net Chat http://cutesoft.net/ASP.NET+Chat/default.aspx
Web Messenger: http://cutesoft.net/Web-Messenger/default.aspx
asp.net wysiwyg editor: http://cutesoft.net/ASP.NET+WYSIWYG+Editor/default.aspx
asp wysiwyg html editor: http://cutesoft.net/ASP
asp.net Image Gallery: http://cutesoft.net/ASP.NET+Image+Gallery/default.aspx
Live Support: http://cutesoft.net/live-support/default.aspx