What I would suggest would be to first completely uninstall and reinstall the plugin from the browser on each computer you are seeing the issue on. I'll link an article below, just in case you need the exact steps:
Please restart the computer after the plugin has been installed again if you see the issue continue after. Just in case you're still seeing the issue after following those steps, please download the Desktop Player for your computer from the link below. This will be right below the Web Plugin options:
The Desktop Player has been approved for viewing on each unit; you will just not be able to make any modifications (remote changes, factory resets, et cetera) from this player.
Also, since you're using a bookmark on the browser, I would suggest deleting this, clearing all of the cache and cookies on the browser, and accessing directly from newwatch.slingbox.com instead. If this works, you can create the bookmark again. I've seen this kind of thing a lot in my own home since I use so many bookmarks. Sometimes, the page coding changes or something like that and it makes the bookmark error out.
I hope this helps!!