We'd need more information in order to help.
* What version of DSpace 7 are you using? (7.0, 7.1, 7.2?)
* How can this error be reproduced (please provide step by step instructions on which page you are accessing when this occurs)?
* Have you customized your DSpace code? This particular error seems to imply you may have added some custom code which is causing the problem... but it's hard to say for certain.
Tim