What URL is returning that response code?
Actually it's from the access log in JBoss, and it's in production environment so I haven't access to that env. Another thing to consider in this is that there are Apache's up front that receives all client requests, and the routes the original req's to one of the JBoss AS.
The client request is directed towards a jsp page where the querystring parameters is URLDecoded and the populated into a Domain object which is then submitted to a DB.
Below is two example lines from the Apache's Access log, the first one is before a migration from Bea WLS to JBoss (at the backend) and the second is after the migration.
81.YYY.48.80 - - [06/Oct/2010:22:51:33 +0200] "POST /sendForm.jsp HTTP/1.1" 200 3395 "-" "-"
81.YYY.48.80 - - [07/Oct/2010:21:02:57 +0200] "kindOfUse=3&hiddenInfo=&returnURI=&name=Birgitta&phone=08-53686463&email=birgitta.ahlin%40capgemini.com&companyName=TEST+KUND&orgNr=004071-0006&ddUsers=&ddCities=&ddArea=PA&description=Kontakta+via%3a+Telefon+Ang%c3%a5ende%3a+testar+skuggformul%c3%a4r+K%c3%a4lla%3a+http%3a%2f%2fwww.myCompany.se%2fforetag%2fprodukter_tjanster%2fproduktomrade%2fvaxlar%2f%3fsl%3dforetag_produkter_tjanster_produktomrade_vaxlar&caughtFromURL=GET /kontaktamig/sendForm.jsp HTTP/1.1" 501 2627
Isn't it peculiar that there are no method (GET or POST) at the beginning of the second one???