What's wrong with my launchctl config?
I'm assuming the paths to the python script and its parameters are valid, otherwise you'd most likely be seeing errors in the Console instead.

The last time I saw that error was because there were spurious characters in the plist, e.g. extra spaces, causing syntax errors and therefore making it fail to load. If you run plutil -lint on your plist, this will check the syntax for you and also handily report back the line on which the error occurred.

plutil -lint com.sickbeard.sickbeard.plist

If that doesn't help, the (OSX) How To Start SickBeard at Login or boot on OSX thread on the sickbeard forums has a slightly different plist to yours (sickbeard running from /usr/local as opposed to /Applications - so it could be a permissions thing) and also some people with seemingly the same problem as you