All those Registry settings relate to the proxy settings for the SYSTEM account. There was a bug fix in SEP 11 RU6 that was supposed to fix this. Looks like it's back.
For me clearing those settings & restarting SEP was good enough. I did not have to restart the machine (which also is a pain for servers).
The other thing to remember, while this fixed your problem, it certainly is not the only reason why clients would appear offline.