Sys.WebForms.PageRequestManagerServerErrorException: An unknown error occurred while processing the request on the server. The status code returned from the server was: 500

Hello,
I have an application that sends XML data to a webservice.This web service do a long running task because it inserts data into database and he downloads pictures, transform them and send them to a directory usign a second web service. This architecture is dupicated on the web server to allow me to test with false data, and not polluting production database.
If I test the "test" application, everything runs well (except an Ajax timeout, but il will take part of another post).
If I test the "production" application, after some minutes, I got this error: Sys.WebForms.PageRequestManagerServerErrorException: An unknown error occurred while processing the request on the server. The status code returned from the server was: 500 and everything stops.
As both application are the same (they're copies) and runnig on the same server, I don't understand what could cause this strange behaviour.
Could ...