After a reinstall of the Oracle database and installation of a higher version of Tomcat a full export of the former 2.1.5 database was applied (without any problems). When trying to add a new user an exception is thrown stating that a ORA-0001: constraint violation has occured. The involved constraint is the primary key of the OS_USER table which can not be disabled and has dependencies to other database objects.
All allready registered users are able to work as usual.