This is the section you are referring to, yes?
'Unable to Connect to Simulator' or 'Despite Our Best Attempts...' or other connection messages
At times, when attempting to log in, you will get a message like “Unable to connect to simulator”. Unfortunately, this message can be the result of several different things. This page covers some of the more common - and more rare - cases when this might occur.
Your Internet Connection Is Down: Check whether you are able to connect to the 'net using a web browser; try to use it to visit a known website. If that fails, then the first thing to try is to unplug your modem/router for at least a full minute, then once plugged back in, allow then to go through startup, after which reboot your computer. If after this you are still unable to connect to the Internet with a web browser, you will probably need to contact your ISP to have your connection checked.
Your Router and/or Modem Has Problems: Repeat the same things as above.
Your Firewall Is Preventing the Connection: Try disabling your firewall software completely and then try to log in again. If you are successful, then you need to allow the viewer access through the firewall. Some tips on how to do that are here: Second Life wiki: Configuring Your Firewall.
Domain Name Server (DNS) Issues: Users of Firestorm may get “Unable to connect to simulator”, or a message specifically saying that there are issues with the DNS. In that case refer to the DNS Failure page for how to proceed.
Region Is Down: It is possible that the region you are trying to log into is down. So try again but select a different region name at the log in screen. Some regions you can try: Hippo Hollow, Aich, Hatton, Pooley.
Group Tag Preventing Login: There are cases when your active group tag will prevent log in - most usually when the tag contains unicode characters (though there have been isolated cases where even standard Latin characters have prevented log in as well). If this happens, you can attempt to log in using a text based viewer such as Radegast or MetaBolt, and change your active group in that viewer (preferably to None), log out and then log in with your usual viewer.
Should that not be an option, you will need to find a way to get word in-world, to the group owner, and have them either edit the group title tag or, should that not be an option, eject you from the group.
If that too is not a viable option, then the only other recourse is to file a support ticket with LL and have them change your active group.
This is a server issue, and is reported in this LL JIRA.
Unknown Settings Issue: Try clearing your viewer settings: How to Clear FS Settings. You can make a settings back up first (see How to Make a Settings Backup) – but be sure to test on cleared settings before restoring your backup.
Kaspersky password manager: If login in fails on the first attempt only with a message like “Despite our best efforts…”, and you use Kaspersky password manager, reset your login info for Second Life in the password manager, then try again. (Information provided by Valeries Muircastle.)