PUT request returns HTTP 400 Bas Request in Spring Mvc - only occasionally

I have a strange issue using Spring mvc: while I'm trying to update one of my resources via PUT, it returns 400 Bad Request for the very same request - but only occasionally.

In Firebug, I use right click "resend" function, so assume all my PUT requests are completely the same. I also set the log level to ALL for org.springframework.web to find out what's happening on the server side. In case of the response is 200 OK, everything seems fine:

2015-04-10 10:40:20,794 [http-8443-exec-12] TRACE servlet.DispatcherServlet:1048  - Bound request context to thread: SecurityContextHolderAwareRequestWrapper[ FirewalledRequest[ org.apache.catalina.connector.RequestFacade@321fa1b9]]
2015-04-10 10:40:20,795 [http-8443-exec-12] DEBUG servlet.DispatcherServlet:845  - DispatcherServlet with name 'springRestAPI' processing PUT request for [/rest/sites/25]
2015-04-10 10:40:20,796 [http-8443-exec-12] TRACE servlet.DispatcherServlet:1101  - Testing handler map [org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerMapping@6c9ddcc4] in DispatcherServlet with name 'springRestAPI'
2015-04-10 10:40:20,796 [http-8443-exec-12] DEBUG annotation.RequestMappingHandlerMapping:297  - Looking up handler method for path /sites/25
2015-04-10 10:40:20,797 [http-8443-exec-12] TRACE annotation.RequestMappingHandlerMapping:335  - Found 1 matching mapping(s) for [/sites/25] : [{[/sites/{siteId}],methods=[PUT],params=[],headers=[],consumes=[],produces=[],custom=[]}]
2015-04-10 10:40:20,797 [http-8443-exec-12] DEBUG annotation.RequestMappingHandlerMapping:302  - Returning handler method [public void something.SiteController.updateSite(long,something.resources.UpdatedSiteResource,javax.servlet.http.HttpServletRequest,javax.servlet.http.HttpServletResponse)]
2015-04-10 10:40:20,797 [http-8443-exec-12] TRACE servlet.DispatcherServlet:1141  - Testing handler adapter [org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter@5ec2c20d]
2015-04-10 10:40:20,798 [http-8443-exec-12] DEBUG mvc.WebContentInterceptor:146  - Looking up cache seconds for [/sites/25]
2015-04-10 10:40:20,798 [http-8443-exec-12] DEBUG mvc.WebContentInterceptor:158  - Applying default cache seconds to [/sites/25]
2015-04-10 10:40:20,799 [http-8443-exec-12] DEBUG annotation.RequestResponseBodyMethodProcessor:135  - Reading [class something.resources.UpdatedSiteResource] as "application/json;charset=utf-8" using [org.springframework.http.converter.json.MappingJackson2HttpMessageConverter@4de52e6]
2015-04-10 10:40:21,280 [http-8443-exec-12] DEBUG servlet.DispatcherServlet:1018  - Null ModelAndView returned to DispatcherServlet with name 'springRestAPI': assuming HandlerAdapter completed request handling
2015-04-10 10:40:21,281 [http-8443-exec-12] TRACE servlet.DispatcherServlet:1058  - Cleared thread-bound request context: SecurityContextHolderAwareRequestWrapper[ FirewalledRequest[ org.apache.catalina.connector.RequestFacade@321fa1b9]]
2015-04-10 10:40:21,281 [http-8443-exec-12] DEBUG servlet.DispatcherServlet:996  - Successfully completed request

But when the response is 400, there is nothing in the log: it does not even reaches the DispatcherServlet!

Any idea, how to proceed with this?