Need help: Error saving action

Hello,

My site uses SMTP Post and for some reason I keep getting error messages whenever my site sends an email to a customer. Here is the error:

Error saving action: ActionScheduler_Action::$args too long. To ensure the args column can be indexed, action args should not be more than 8000 characters when encoded as JSON.

I also use mailgun and I can see every email my site has sent and when this error occurs Post SMTP thinks the customer did not get the email. But they did because i can see the email log in Mailgun. So Post SMTP will keep sending the same email to the customer.

How can i fix this issue?