What causes an Unavailable Character Encoding Error?

I hope someone can help us solve this mystery. We use the Technical Communication Suite 2 for single sourcing with FM 9 and RoboHelp 8, with a WebHelp output. We also create a PDF and use conditional text for online only and print only, among other tags.

Now, when one of our writers creates the online help and PDF, I can no longer open the FM source file.

I get this error: "Document named x uses unavailable character encodings. If you open this document, some characters may appear as nonsense text. To reformat the documents using available fonts, click OK."

I click cancel because I don't want to run the risk of corrupting the file. When a third writer opens the file, makes a change, saves, and closes it, I can then open that same file without the error message.

I'm stumped. Is it a font difference? Some sort of setting in writer #2's FM? What could cause this? Your help is much appreciated. After our last build I had to ask writer #3 to open all of our FM files, change and save them, and then close them so I can open them again.

So there is a workaround, but obviously I want to find out the source of the issue.

*I also posted this in the General FrameMaker forum. It was suggested I place it here as well.

Writers 1 & 2 use the same version of FM & RH. Writer # 3, who "clears" the file, has an earlier version of FM and doesn't have RH. She should upgrade to our version, but doing so will break our fix.

Help?