How can I fiix appearing edges

I am still new to Blender and ran into some problems trying to use the freestyle render features. I have a mesh showing bare stairs:

When rendered, an edge is detected which should not be there. It is on the front, not the back.

I know this information might be insufficient but I don’t know what else I should post. What could cause this problem? I think the mesh itself might be messed up but I cannot figure out a way to “clean” it.

How can I fix / clean up the rendering / mesh? Any suggestions or guesses?

Thank you for taking the time to help me!

Go into Edit mode and press Shift+Ctrl+Alt+M or go to Select and choose Non Manifold.

You should see that edge appear. Delete it.

That selects these edges:

The critical edge is not selected (although others which cause problems, too).

Solved the problem with a remesh modifier. Not a perfect solution, but it works.