If you dial a number on the number block or through your contacts, the first call attempt mostly fails, saying "Call finished" straight away. The second attempt usually works then. It's very annoying when you have to insert the whole number again, since the manual dial is lost then.
Some research finally showed, that Avast Anti-Theft is the cause for this behaviour, and that this happens on all Galaxy S5 with Lollipop.
See
http://www.android-hilfe.de/samsung-galaxy-s5-g900-forum/656922-anruf-telefonate-aus-direktwahl-oder-dialer-werden-umgehend-beendet.html This forum thread says, that the Avast Support has confirmed this bug at beginning of march, saying that it will be fixed in the next update.
Since then, over two months and several updates have gone by, but the bug still exists. Very frustrating....
