Thanks John,
Nope, I don't have access to the server logs. The 'new guy' is controlling that, and I can't get access.
I asked, and he was just looking in the IIS logs.
I don't know anything about MS FTPS server (if it's part of IIS, how it's installed, where it's configured...) so I couldn't help him by suggesting where to look.
He finally ripped it out, and installed the FileZilla server, and now it's functioning, so we are beyond that point for now. (until a customer tries to use that server for their installation)
Also, John, I have another question posted which I haven't gotten any help with.
How to control the protocols (and along the same line, how to control the ciphers... 'best practice' is to not use all 34 of the NIST ones, but reduce them to perhaps 10 or so)