I am receivin this general error.

Error(s) occurred while submitting the page. The error details are:
An exception was encountered when attempting to populate default value: (System.TypeException)Illegal assignment from List<Account> to Account

This is in our sandbox environment. We were able to use our sandbox without any issues a few days ago. We didn't make any change to our code. Today, we received this error. We ran a debug log and couldnt find anything either. What is going on? 

I am receivin this general error.

Error(s) occurred while submitting the page. The error details are:
An exception was encountered when attempting to populate default value: (System.TypeException)Illegal assignment from List<Account> to Account

This is in our sandbox environment. We were able to use our sandbox without any issues a few days ago. We didn't make any change to our code. Today, we received this error. We ran a debug log and couldnt find anything either. What is going on?