Yes, I thought of Aphyllanthes, indeed it is very close, but the locality (moist alpine meadow at 2000m) does not fit and my plant has one deep furrow while Aphyllanthes has several, and less deep. Still very close to Aphyllanthes.... Also, I haven't found any Dianthus/Arenaria/etc. which comes close... I did not save the seedpods, unfortunately.
Thanks for your suggestions. It seems that only forcing it into flower will answer the question. But it hasn't flowered for three years now... We'll see; I let you know.
Hans