[Kino Haitsma]

Thanks for all these comments and tips!
I talked to an editor who said the problem was audio related, because I said I got a RED render line each time I imported an audio track.
Also I read online that you don’t need MPEG streamclip to convert the iPhone footage – FCP can do it from the Media Manager.
So I changed my workflow a bit.

Import iPhone clips into FCP.
Use media manager to recompress/change codec to ProRes 422 LT 1920×1080 at 30p.
(The only thing I find strange is after having converted the clip, and looking at the settings, it says 29,79 – not 30)
I find that strange because the codec clearly says 30p. Anyone has clarity on that?

Anyway, the big mistake I made was to simultaneously record the audio in Ableton, at 44.1…
I did a test recording at 48 (same as the ProRes default) import the audio, and boom, no render line at all…

I also wonder why would one use MPEG streamclip, when FCP does a good job too?