Hi Anders;
The error that are you facing it's a value for the new z-field, because maybe the user types some special symbol into the new string field.
In large text field you can insert those special symbols, but the string doesn't permit, so does it is the error.
You can face this error when the batch it's running or when you open the portfolio item.
I suggest that you make a control to check the value that users type into those large fields, or convert those values.
Best regards,
Mariano