AutoResponder Rule Editor only shows the URL match field, and the "Test..." option.
The combo to choose a pre-defined response and the Edit Response button don't show up (and the panel cannot be resized)!
This seems to be new in v5.0.20181.14850, as it never occurred with older versions.
FWIW, you may be able to workaround this in a very hacky way in C# FiddlerScript, add the following to Handlers:
Save the script, then click "FixPanelHeight" on the Tools menu. Adjust the values for the Height and Top as appropriate.
Unfortunately, I've heard of this problem popping up in occasionally before, usually on systems in China where the user doesn't have Fiddler's default font. If you play with the Font/FontSize inside Tools > Fiddler Options > Appearance *and restart* Fiddler, is there any change?