This is probably due to a an attachment, which at one point was linked to a CR, but has since been deleted. When a user opens the CR the error is thrown in the server log. To prevent the error from continuously appearing in the Server log, create a dummy file, basing the name of the file on the attachment ID seen within the error message. For example:
The dummy file should be named, based on the attachment ID, "000203F9" and this will need to be located within the directory
If files exist within this directory, make a copy of any of the files and rename it based on the attachment ID. When the CR which contains this attachment is opened there will be no error in the server log.
To ensure that the attachment does not contain any information, right click and edit the file with Notepad and remove any information from it. As long as the file exists in the specific location and named with the appropriate attachment ID the error will not re-occur.