Directory Listing Denied on 'more'

When I use the truncation option in a text field to show "more" after a specified number of characters, the full text of the entry should be displayed in a new window. This works fine for most of the records in the table. However, when "more" is clicked in one record, the new window shows an error message that reads:
Directory Listing Denied
This Virtual Directory does not allow contents to be listed.
The text in the field doesn't contain any special characters or other unusual things.
Can you tell me why this occurs and how to fix it?