Email attachments will not bed displayed if the SiteID value is 0. The page getattachment.aspx cannot render if the URL contains "siteid = 0".
This can be caused by two things:
There is a known bug with the Extended XML Logging, if the SiteID property is unset or set to 0 the value is then retained and used for any newly logged Cases.
NOTE: This bug has been fixed in versaSRS v7.2.0 if you are not running the latest version we highly recommend you update your versaSRS.
Someone manually changing the value in the database.