Oiwan Lam reports widespread disruption for users of Freegate, the popular circumvention software in China:
According to the RFA report, users from several provinces across the country have encountered similar problem and they believe that it is due to the upgrade of Great Fire Wall. Apart from the Freegate, when running UltraSurf and FreeU the same error message appeared.
Freegrate appears to have quickly adapted. It's hard to tell from the descriptions, but it may be a wider set of Freegate relay servers (and other censorship circumvention software) was blocked.
Circumvention software works by using a not-well-known computer to send your data to, in a format that you hope the blocking authority won't be able to decode. The vulnerability is always that identity of that relay will be discovered, and all traffic, no matter how innocuous looking, will be blocked.
One other possibility is the GFW can now recognise the particular patterns of data sent by these circumventions systems. Or it could be just a temporary glitch, unrelated to such cleverness. Such is the nature of the Internet weather.
Whatever it was, Freegate fixed it with a software update, and I bet those other services were right on it, also.