I received the following error on a website I visit often, after right clicking the KeePass saved entry and clicking Download Favicons:
Could not download favicon(s). This may be a temporary problem so you may want to try again later or post the contents of this error message on the KeePass Favicon Download forums at http://sourceforge.net/projects/keepass-favicon/support. Technical information which may help diagnose the problem is listed below, you can copy it to your clipboard by just clicking on this message and pressing CTRL-C.
SendFailure: Error getting response stream (Write: The authentication or decryption has failed.): SendFailure:
I thought this was possibly just the website I was visiting, because it was Cryptsy.com and while viewing their website source I did not see a link in the <head> for the favicon.ico so I assumed they may use some script to insert it. When I just signed up at SourceForge, I attempted the same thing after saving my log in info, and received the same error. I just vied their source and realized that they both have something in common, that they use the Pictos and Picons icon added through the css scripting for those special icons. I don't know a lot about them, never used them, only know what I have seen in sources. So, if that is why the favicon is not able to be downloaded, it would be great to come up with an update which can read the src url which contains the favicon from the icons css.
Besides that I love the plugin, it makes seeing and choosing the entries a lot faster.
I don't think there is any way to display a favicon based on CSS. If I'm wrong then it's definitely something that is only supported in recent and specific web browsers and therefore unlikely to be used in the real world without the site also offering some kind of fallback option to all other browsers.
In the case of sourceforge, the favicon.ico file is in the usual place but they have just omitted the metadata in the page source. That's not a problem for this plugin and I see that the Sourceforge icon is downloaded correctly for me.
You might find that another correlation with the icons that don't download is that they are for HTTPS websites.
I've never seen the error message you've posted so I'm not certain what could cause it but I can make some guesses: A transient problem with the website that has now gone away; an intermediate proxy is corrupting HTTPS communication from your machine; a corrupted .NET installation; a corrupted Windows installation.
Thanks for the feedback; glad you find the plugin useful.