Thanks @davep and @DeltaMikeCharlie - I did have the version that is the current default with Raspbian OS. But you were right, I registered the TVHeadend repo and updated the version, now I'm on 4.3-2715-g80729d2f6. And the timer works. So far so good!
But I have to say I'm a bit disappointed with the timer functionality. It's very much geared towards the "record my favorite show every Monday" use case, and not really suited for "I want to tape world cup football games". For example I can't select a date for the recordings - only weekdays.
I'm a bit stumped now, the EPG way isn't reliable and the manual way can't do single-shot recordings.