While working on some of my stencils I got the following:
An error (100) occured during the action Open Stencil
Visio cannot open the file because it's not a Visio file or it has become corrupted.
I was trying to open about 10 stencils at the same time but I'd been doing that all morning with no problem. So, I thought maybe memory resources are getting low so I close down Visio and restart the computer. Made no difference.
Next I open each file individually: all open correctly.
Ah, now I see that two of the files have the same file name except one is a .vss and one is a .vsx. Try to open just those two and I get the error and neither file gets opened.
All the documentation states that this error is caused by the file being corrupt or it not being a Visio file, but it seems to be just a more generic open file error but someone has decided to gold-plate it by stating some specific reasons that indicate that they are the only reasons for the error.
I hope this is useful to someone.