You can avoid this only with a grid setting of 1/1. I suspect that FontLab rounds the values at export.
@mekkablue Just checked it, in Fontlab I work with a 0/0 grid and when exporting this is automatically rounded to a 1/10 grid (coordinates have one digit after the decimal point), so that the ‘error’ can indeed occasionally occur at some curves. I solved this in Glyphsapp per curve by tweaking the shapes and coordinates and check the exported .otf again.
I have learned a lot from this project. The next font will be a more ‘normal’ font with grid 1/1 for sure 
Thank you so much for your help!
I had a look at the file. They round to a tenth of a unit. The most important thing is to move the starting point so that the last segment is a line.
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