PDOException Error upon saving node with computed field
I encountered a similar error on my current project and fixed it by adjusting the size of the related field.

Try using PhpMyAdmin to view the 'field_computed_label_value' table and then select the structure tab.  It will list the type as varchar(###) where ### = the character limit for the string.

Try increasing the size of the type for the field value by selecting the 'change' icon for the value row and then adjusting its length.

Remember to back-up before trying this in case it does not solve our problem.