No SecurityManager accessible to the calling code

The security context is not bound to the thread context.
The application uses an ajax periodical updater to often refresh some zones. But the user is enabled to interact with the application, so it happens, that two simultaneous requests arrives at the application server. This often produces
ERROR 2010-05-15 23:33:08,030 (SecurityModule.java:253) - Error handling SecurityException
org.apache.shiro.UnavailableSecurityManagerException: No SecurityManager accessible to the calling code, either bound to the org.apache.shiro.util.ThreadContext or as a vm static singleton. This is an invalid application configuration.
at org.apache.shiro.SecurityUtils.getSecurityManager(SecurityUtils.java:124)
at org.apache.shiro.subject.Subject$Builder.<init>(Subject.java:616)
at org.apache.shiro.SecurityUtils.getSubject(SecurityUtils.java:57)
at org.tynamo.security.services.impl.SecurityServiceImpl.getSubject(SecurityServiceImpl.java:37)
at org.tynamo.security.services.impl.SecurityServiceImpl.isAuthenticated(SecurityServiceImpl.java:42)
at $SecurityService_1289de25571.isAuthenticated($SecurityService_1289de25571.java)
at org.tynamo.security.ShiroExceptionHandler.handle(ShiroExceptionHandler.java:74)
at org.tynamo.security.services.SecurityModule$3.advise(SecurityModule.java:250)