I've been using SkyPlayer on a mac for over 2 years now and its mostly worked great, until about 4-5 months ago when I started getting a c1501 error, screenshot here http://www.megabytesolutions.co.uk/SkyAuto2.jpg. I left it alone for a few months as I figured it was a sky problem at they'd sort themselves out.
Then about a month ago Sky announced Sky Go, their new implementation of Sky Player. I figured that would be that, so I fired up Firefox and logged in. I got the same error and with a quick google found that many others where having the same issue.
I tried sky's fixes but got nothing. I managed to get the issue fixed by doing the following. I'm not saying this works 100% because I haven't been able to replicate the problem again but...
Macbook - Running Snow Leopard
1. Uninstall Silverlight completely (http://forums.macrumors.com/showthread.php?t=474827)
2. Install Silverlight 4
3. Go into Silverlight settings and disable DRM download management (Playback tab)
4. Close preferences
5. Try and play video again
6. Go back into Silverlight settings and enable DRM download management (Playback tab)
7. You should now be able to play the videos without error.
Now you just got to decide what to watch...