That is a similar error but not related to the captcha, you should clear the log then cause the captcha error then look in the log again and you will probably see a similar error for CaptchaImage.ashx
It means that .ashx extension is not configured correctly on the server or IIS is configured to return a 404 without letting .NET code handle the request for .ashx files that do not exist.
It is true that the file does not exist but the file does not need to exist because it is mapped from Web.config like this:
<add name="SubkismetCaptchaHandler" verb="*" path="*CaptchaImage.ashx" type="Subkismet.Captcha.CaptchaImageHandler, Subkismet" preCondition="integratedMode"/>
and csshandler.ashx is mapped like this:
<add name="CssHandler" verb="*" path="*csshandler.ashx" type="mojoPortal.Web.UI.CssHandler, mojoPortal.Web" preCondition="integratedMode"/>
so it is a server configuration issue and you may need to get help from your host to resolve it.
see also this related thread, it has a screen shot from IIS 6 showing the extension mappings.
Also I can tell from the paths in your error that you are using a very old version of mojoPortal, it would be better to use the latest version if possible.
Hope it helps,