When supplementary Unicode characters like 𝒟 are in the metadata of an item this will break the XMLUI and OAI webapps.
In previous DSpace versions with Xalan 2.7.0 this is not the case. The error was introduced with Xalan 2.7.1 and is part of 2.7.1. See: https://issues.apache.org/jira/browse/XALANJ-2419 for the bug report and this blog post for a nice explanation: http://www.dragishak.com/?p=131.
To recreate this bug just import the attached zip file to a DSpace 6.x instance