I have a model with several flex bodies. This message doesn't point directly at the problem mnf, but I have my suspicions it's one for a handed part where I mirrored the original mesh to get the other side. anybody else experienced this?
WARNING: Edge with no mate found.
The shell may not be water tight.
If the shell is from a coarsened Flexible Body,
it may experience difficulty when used in a CONTACT.
I see this often when running the ATV tracked vehicle toolkit on particular road surfaces (which based on visual inspection, are quite likely not water tight!). Right after the exact same message you describe, the msg file lists out the .shl FILE name, attached PART, and corresponding ELEMENT. No flexbodies are used in these particular simulations.
I'd be interested to know if you tried replacing the mirrored flexbody with a rigid part, and still get the same error message?