The fact that your cert's filename has a ".txt" extension is strange...if it's letting you import a text file, it might be corrupting cacerts. Always make a backup of cacerts before modifying it. That way, you won't have to reinstall the JDK if something goes wrong.
Also, one minor note: The "-import" parameter is deprecated in more recent versions of the JDK.
You should use "-importcert" instead (but "-import" still works).