Why can't I edit variables, and edit box displays only 'Page Count' or a blank character?

I have FrameMaker 7 docs that I'm now editing in FrameMaker 9 (9.0.4). I need to wipe out some old variables and update some definitions. When a select a user variable in the Variables list and click the Edit button, in the Add/Edit Variable box, the Name box is blank (or has a single invisible character highlighted).

When I try to do the same thing with a system variable, such as Table Sheet (or any other one), the Edit System Variable box appears but the Name field always displays "Page Count" -- not whatever variable was selected.

It seems there's some corruption or problem with variables in this document. Could this be a character set problem? Some variables were defined long ago with German text. The Table Sheet variable is defined as "(Abschnitt <$tblsheetnum> von <$tblsheetcount>)" for example.

I applied all FrameMaker 9 updates today, and this behavior hasn't changed. According to the Help, I'm doing this correctly. Is it user error, Frame weirdness? Is there a way to fix it by going the MIF route?