I would start over.
In the Finder go to Preferences->Advanced and make sure Show all filename extensions is checked.
Export a new keyword file from your fist library to the Desktop
Open the Desktop in Finder and locate the keyword file it should like like this (of course with thename you gave it)
Notice the lack of an extension while the other files all show an extension. Now select the keyword file and do an File->Get Info on it. Go to the Filename and extension section, lace the cursor at the end of the name and add .txt.
You will get an alert from finder asking you if you really want to change the extension
If you don't get the alert something is not right. Now try the keyword file in Aperture again.
Post back.