I updated the browser plugins on my several PCs (Windows XP, Vista, and Windows 7) yesterday in the Internet Explorer and Mozilla Firefox browsers, and all are working at this time with my Slingboxes. Have not tried this with Safari or Google Chrome browsers, however, as I rarely use either of those browsers. Slingboxes also work with the standalone Slingplayer for PC software on each of these PCs.
It seems that you are having some issues with your Slingbox, since you are not able to connect using different computer OS's and browsers.
We recommend you to check the following link. It will allow you to get the information about the recommended web browsers, according to your computer and OS.