Well, ever since version 10 replication has increased, and in some odd instances the added network usage has effectively prevented people from joining. However, this issue is so rare it affects about 1% out of all people, and can usually be overcome easily even if it hits you.
Before you read on, you should check if the server actually is set up properly, because if you just have a slow computer the network timeout may simply be too low (read advanced network settings before you consider this issue).
If you're certain you have checked back with the server admin and your client settings, then see if the following symptoms may apply:
Pitbull and UnLeaded have determined that ONE definite troublemaker are some anti-virus programs running in background - Kasepersky in this one particular case, where UnLeaded suddenly could connect to any 10B server once he disabled it.
Since I do not know whether or not this is the ultimate solution for all people with this problem, I leave the rest of the document intact as it was before so you can check what has been tried and determinedd so far.
The precise cause is not one single issue, but a combination.
Together with
The above hints led us to the conclusion, that this rare combination of circumstances cannot be narrowed down to one single point of correction. At this time, I do not even know if I can get it fixed ever, because the new variables are needed for features in the mod. Then again, since it is so rare and can be corrected simply by using a new fresh install (either of Windows, Unreal or both) or by using another computer, the question is quite valid if servers should be kept downgraded.
This particular issue will be revisited, as I will redesign login and possibly the way/location I do put additional replicated data. For now, try reinstalling or using a different computer if you have one, in case you are positive to be a victim of the issue. In any case, keep a backup of your old Unreal somewhere so you can easily retrieve map, cache and config files when you need them again.