Is Adblock (Plus) safe or a security risk?

It is not. This is a FUD (fear, uncertainty, and doubt) campaign by GMX because they want to display their ads. There is absolutely no security risk from the mentioned ad blockers. They added some crapware to the list to make it look more legitimate. Of course, such campaigns are very unusual, especially from such a big and well-known company like GMX. Unfortunately, I have no English source at hand (because it's a German only campaign) but since you speak German you may want to read this article at heise.de.

  Update #1: United Internet, the company behind GMX, received a lot of criticism for misleading customers by falsely claiming that there is a security risk on their PC. The Wall Street Journal (German edition) named the warnings displayed on GMX and the site they link to a "scare campaign".

Update #2: GMX now says that they will no longer display the link when you use ad blockers but will still display it if you use crapware that injects adverts, the list at the site http://www.browsersicherheit.info/ has been updated accordingly and now lists only a small collection of crapware. This list is by no means complete so it is not a reliable source when you want to know if your browser has crapware installed. However, United Internet still maintains its position that they do not want users who visit their sites to use ad blockers and said they will develop other anti-blocking methods in the future (German source).

Is Adblock Plus safe for android?

  Adblock Browser is fast, secure and free. Crazy fast, secure and free ad blocker browser for Android that blocks ALL ads. Surf the web without annoying ads. Adblock Browser is fast, secure and free.dblock Plus is also available for Android devices. … To install Adblock Plus, you will need to allow app installation from unknown sources: Open “Settings” and go to “Unknown sources” option (under “Applications” or “Security” depending on your device)