When I export my SketchUp STL model to Cura it looks good in solid mode but in the layers mode some holes are filled up. To find the reason why, I broke down the original model by deleting the parts that come out ok, and ended up with a block 50x100 mm with a hole 30x20 mm in it. This simple stucture looks manifold, no double lines etc, and does not have red parts in Cura xray.
Cura shows the model as expected, but in layers mode the hole is filled up. I spent many hours to find out, but so far my only solution is to start over again drawing a complete new model and even then this happens again.
I'm using SketchUp 2019, Cura 4, Meshlab 2020
What goes wrong here? How can I repair this with Meshlab?
STL file available on request.