Hi Wim,

This is a strange one, looks like a tolerance thing with Planar surface, just a guess, Michael will have to look into it.
The work around for now, this is why I find it strange, I un-trimmed the Planar surface, you do that by selecting the surface then the edge press CTRL+A then Delete, then I used Edit > Trim to trim the surface again, do that for both sides and Join, it should now be a solid.