When an error like this occurs to any custom created dunning roles (manager / director) note down the script id from the error screen
Open any script and change the internal id and click enter by going to customization> script> scripts
Now the Dunning Queue SU (SS2) script will open
go to deployment subtab
open the Dunning queue ss2 -email
edit the record and add the custom role as audience
save the record and try again the email sending queue page.