NullPointerException in Spring Framework (or what?)

I have run into a very weird problem, and I have simply no idea, where to start. In our working application, one of our pages seems to fail kinda arbitrarily - that is, sometimes it works, and sometimes it doesn't.

The web application is running Java 6, Spring 2.5.6 and Hibernate 3.2.6. Furthermore, we are using Javax.servlet version 3.0 (might be relavant).

The problem becomes hard, because of a very limited stacktrace, which doesn't seem to involve any of our custom code (all our classes start with com.isworld.*).
The error occurs AFTER loading the page, when the form is submitted and the request is beeing processed. See stacktrace below:

org.springframework.web.util.NestedServletException: Request processing failed; nested exception is java.lang.NullPointerExceptionorg.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:583)org.springframework.web.servlet.FrameworkServlet.doPost(FrameworkServlet.java:511)javax.servlet.http.HttpServlet.service(HttpServlet.java:637)javax.servlet.http.HttpServlet.service(HttpServlet.java:717)org.acegisecurity.util.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:265)org.acegisecurity.intercept.web.FilterSecurityInterceptor.invoke(FilterSecurityInterceptor.java:107)org.acegisecurity.intercept.web.FilterSecurityInterceptor.doFilter(FilterSecurityInterceptor.java:72)org.acegisecurity.util.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:275)org.acegisecurity.ui.ExceptionTranslationFilter.doFilter(ExceptionTranslationFilter.java:124)org.acegisecurity.util.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:275)org.acegisecurity.providers.anonymous.AnonymousProcessingFilter.doFilter(AnonymousProcessingFilter.java:125)org.acegisecurity.util.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:275)org.acegisecurity.wrapper.SecurityContextHolderAwareRequestFilter.doFilter(SecurityContextHolderAwareRequestFilter.java:81)org.acegisecurity.util.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:275)org.acegisecurity.ui.rememberme.RememberMeProcessingFilter.doFilter(RememberMeProcessingFilter.java:142)org.acegisecurity.util.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:275)org.acegisecurity.ui.AbstractProcessingFilter.doFilter(AbstractProcessingFilter.java:271)org.acegisecurity.util.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:275)org.acegisecurity.ui.logout.LogoutFilter.doFilter(LogoutFilter.java:110)org.acegisecurity.util.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:275)org.acegisecurity.context.HttpSessionContextIntegrationFilter.doFilter(HttpSessionContextIntegrationFilter.java:249)org.acegisecurity.util.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:275)org.acegisecurity.util.FilterChainProxy.doFilter(FilterChainProxy.java:149)org.acegisecurity.util.FilterToBeanProxy.doFilter(FilterToBeanProxy.java:98)org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:96)org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:76)root causejava.lang.NullPointerException

This is all there is to the stacktrace.

I do not expect any of you to come up with a solution to this problem, but hopefully, you can point me in some direction - I have no more ideas...