various notes error list can be seen at http://www-12.lotus.com/ldd/doc/domino_notes/rnext/help6_designer.nsf/f4b82fbb75e942a6852566ac0037f284/3981119508a738ac85256c54004d9d07?OpenDocument
in your case following could be the reason
1)from the error message it seems that you are trying to append an item in an array whose upper index is not dynamically extended.
2) may be you are tring to append some text in the text field whose property is not selected as allow multiple values.
pls check as I feel these as probable cause