An error has occurred in <workflow name>

I was receiving an error in my SharePoint Designer 2010 workflow on an approval action.
The error read:
An error has occurred in <workflow name>
I know, very helpful.

There was not anything helpful in the logs either.

I spent an hour trying various things like:

•  Removing task form fields
•  Removing actions I have inserted in the single task behaviour as well as overall task process.

With no luck I finally decided to hard code the assigned to person.
IT WORKS!

After a little investigating I figure out that the lookup person / group I have assigned needs to be returned as “display name” instead of the default, string