Task Creation Failing

Hi,

I have been recently facing unwanted redundancies in a simple autoresponder program setup. When a new person is created and someone fills out a demo form an autoresponder email goes out and a task is supposed to be created. This program was working absolutely fine until recently. 

We were using the auto-assignment rules i.e. "Assign to Sales Owner"  in the Task creation step. (Screenshot below) but getting this error. No unwanted changes have been made in SFDC or in Marketo. The same program setup has been running throughout the past year and no errors or exceptions have been caught until now. Can someone please tell me what is the root cause of this and how to fix this? I am finding no luck troubleshooting this.

Exception:-