Vodafone UK isn’t letting its customers access the website of the Chaos Computer Club (CCC), one of the oldest and largest associations of computer hackers in Europe.

It’s not clear why the mobile operator has blocked the ccc.de website since late last week, but the CCC believes it’s because its site has been misclassified in the Internet filtering system used by Vodafone.

Since its founding in 1981 in Berlin, the CCC has highlighted security risks in technology affecting a large number of people, has exposed government surveillance and has advocated for privacy and freedom of information online. Every year the group organizes the Chaos Communication Congress, the largest hacker convention in Europe.

Vodafone UK did not immediately respond to a request for comment.

For years ISPs in the U.K. have blocked access to websites that host images of child sexual abuse based on a list maintained by an organization called the Internet Watch Foundation. However, in 2013 the U.K. government convinced ISPs to build their own Internet safety filters in order to block sites that host content unsuitable for minors, including pornography and drugs.

The initiative was criticized by human rights and Internet advocacy groups because it allows ISPs to judge what is right or wrong for young people and opens up the door to over-blocking, where users are prevented access to non-harmful and even helpful websites and information.

“When these filters were introduced, their abuse was imminent,” said CCC spokesman Dirk Engling in a blog post Friday. “Today, we are shocked to learn that they not only block access to our site, but also to our conference. We see this as proof that censorship infrastructure—no matter for which reasons it was set up, and no matter which country you are in—will always be abused for political reasons.”

According to a check on www.blocked.org.uk, a website maintained by U.K.-based Open Rights Group (ORG), ccc.de was being blocked by both Vodafone and Three, another U.K. mobile operator, as of Sunday. The Chaos Communication Congress tickets site, tickets.events.ccc.de, was only being blocked by Vodafone.

“Overblocking is a major issue when it comes to filters that are designed to stop children and young people from seeing adult content,” said Pam Cowburn, ORG’s communications director, Monday via email. “When we tested the Alexa 100k websites, we found that almost 1 in 5 sites were blocked. Although many of these may have been deliberately blocked because, for example they are pornography, this high figure suggests that there is a real problem with over blocking.”

Sites that have been incorrectly blocked in the past include political blogs, sites about women’s rights and even a Porsche sales site, Cowburn said. A site that offers support to victims of sexual abuse and rape, aged 16 and higher, was also blocked and so were some sites that belong to charities, she said.

Users can opt out of these filters or can bypass them using technical solutions, but only a minority of them know how to do it, the CCC said. For example, it said its CCC.de server can be accessed directly via http://213.73.89.123/