Simple manifold problem

Here is a pretty simple part. I uploaded it 2 weeks ago and it was rejected for "inverted normals". Strangely, this week I uploaded exactly the same file and the error I get is now "Only manifold objects can be printed."

MiniMagic claims the part is OK. A friend ran Meshlab on the part and found no problems with it.

This file was created by another piece of software, and we don't just want to fix the part. We want to know what is wrong so we can fix the program that exported the file. Can anyone help? Please don't just fix it, can you tell me what is wrong?

Thanks in advance for any help!

Thanks to Arno and Shapeways, my problem has been solved. The mystery is that to most software this ASCII STL file is perfectly fine. Why then does the Shapeways software reject it? As an ASCII file, it contains vertex coordinates using number like 3.615e-003, in other words in scientific notation. The Shapeways software can only handle coordinates of the form 0.003615. Converted to binary STL the same file was fine.

Most everyone uses binary STL as the files are more compact, so probably nobody else will run into this.