
5 answers
As @Nash mentioned, it could be validation rule... Can you try with any of the tools and see it works -
Data loader(EE or UL editions) -
http://wiki.developerforce.com/index.php/Apex_Data_Loader
Excel connector((PE, EE, and UL Editions) -
http://wiki.developerforce.com/index.php/Force.com_Excel_Connector