I am not sure if this is it but make sure both the inbound and outbound message layouts exist.
You can use a viewer to view the suspect Xlate file and in the Prologue will be the specification of the inbound and outbound layouts required by the Xlate.
If they do not exist, reinstate them.
If they do exist and there is no other issue (like someone editting the Xlate manually and hosing it up) you can always go back to the last revision copy of the Xlate that did open successfully.
email: jim.kosloskey@jim-kosloskey.com 30+ years Cloverleaf, 61 years IT – old fart.