I have several entries for which the plug-in fails. One example is http://www.vonage.com/. There seems to be an icon at http://www.vonage.com/favicon.ico, so I think the plug-in *should* work. The error I get is:
A null reference or invalid value was found .
I am running KeePass 2.15 on Ubuntu 10.10 with Mono 2.6.7.
I've just released version 1.6 which is able to download the Vonage favicon. Please let me know if it works for you.