I am using OJS 3.5.0.3 installed using Softaculous. Just recently, the email templates are not loading properly when sending notifications to any users. If I will be sending review results, there is also no options to attach any document shared by the reviewers.
When you check for the Prepared Emails are you able to see that the emails do exist there?
This would be in Settings → Workflow → Emails → Add/edit Templates (under Email templates)
Test more than one workflow. For example, check a reviewer invitation, an editorial decision, and a general announcement. If only one workflow fails, the issue may be specific to that template or mailable.
Are you able to check your PHP error logs and report on any errors found there?
FYI: Softaculous is not an ideal way of installing/updating OJS - it has been known to cause issues for users.