I have created a font in Glyphsapp, and used in InDesign.
But as shown in the image, some glyphs have a thin line in them.
http://postimg.org/image/3u9uucflb/
When I zoom in, the lines disappear…
I guess this is because the glyphs are taken from Illustrator, but what can I do about this?
Clean up and simplify the paths in question, I’d say.
Also, disable autohinting at export.
If the problem persists, you can mail us the file and we’ll have a look.
Thanks for the quick reply and for pointing to that blog post (I had already read that) 
The trouble were indeed the scanned hand-made forms, and converted to paths with Illustrator…
On the intersection between the thin lines and the outlines, I found things like:
-green control points that made an angel (but should be 180°)
-blue control points that created a little intersection in the outlines.
http://postimg.org/image/pzgyvjtlb/
solving these solved the thin lines.
It wasn’t the first time I had used the method of auto conversion of images to outlines, but it was the first time it didn’t work as expected…
I can solve this now, I will just check all control points
aside, it would be nice if Glyphsapp could do a check for this when pasting paths out other programs?
What you can do now already is Layer > Tidy up Paths (Cmd-Opt-Shift-T) for all glyphs. Also, Add Extremes and Correct Path Direction may help. See this blogpost for more details. This helps, but AI paths will remain a problem.
That’s why I usually suggest avoiding AI. Did you use AI solely for tracing scans? Have you tried the Trace plug-in for Glyphs?
Didn’t know that plugin, thanks!
All forms are now ready in Illustrator,
but when this font is ready, I will give this a try