This mesh is not valid.
What does not go in the mesh:
The mesh has 1 edge extremely short.
Is there a way to locate this short edge without using external mesh repair software?
Simon
If you can think of fixing it at the same time, perhaps CollapseMeshEdge can help you?
Other Collapse commands that may be of some help:
// Rolf
The problem is not to fix the short side. The problem is finding it.
_Simon
If GrassHopper is an option, then you can “attach” your mesh to the mesh component (pictured) and look out for the Sphere. Make the SphEre big to start with using the slider, and then shrink it while you zoom in on the tiny mesh edge :
Mesh - Find Shortest Edge.gh (7.8 KB)
// Rolf
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The mesh is too big and unfortunately I have not yet learned how to use GrassHopper.
Too big for what? So it can’t be opened in Rhino?
If you install the GrassHopper plugin, then you type “GrassHopper” in the Rhino command field, and GrassHopper starts. Once started you open the attached file, using the file menu shown in the picture, then right-click the leftmost Mesh component in the picture, and select “Set one Mesh” (and then click on that Mesh in the Rhino viewport) and confirm.
Then GrassHopper will examine the mesh and show a Sphere at the location of the short edge, just like in the picture.
// Rolf
Forgive me, wrong use of my English language. I was able to use the Grasshoper component and locate the short edge. Problem fixed.
Thanks for your patience.
-Simon
Glad you found it. Rhino rocks! 
// Rolf
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