>>Just a supposition:
>>If the new instance fails to initialize properly it may mean that the currently running instance is in some way blocking access to resources that the new instance needs at startup? And if the problem only happens when the site it busy (i.e. with active hits and sessions) then maybe it's just the session that is blocking the relevant resource......
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>It also happens when the site is having almost no user. But, it may well be that a hit is in progress.
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>Could you elaborate more on the session issue? I am not sure what you mean.

Just thinking that if, for instance, the session uses a resource in a shared or readonly mode then it would work fine with other concurrent sessions but if the application at startup required exclusive or write access to that resource it might be a problem.

>>Are you logging (or inadvertently suppressing) any errors that might occur at startup ?
>Where would that be?

I was thinking about errors that might occur and be untrapped in the Global.asax Application_Start() code. Do you have code in the Application_Error() handler that would let you know if an error occurred there ?

>>Also, IIRC the message on the client was something to do with 'waiting' i.e. - *not* 'page not found'. That implies that the site is not completely inactive and that the client is hitting the new instance in a partially spooled-up state ( or, possibly, still hitting the old instance in a partially shutdown state).
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>It is probably what it is. But, if I wouldn't do anything, it would take a very long time before the site starts responding again or not at all.

You probably need to narrow this problem down to more specific scenarios. For example see if replacing the DLL *always" works when there are *no* active sessions. And again, from a problem solving POV, 'a very long time' and 'not at all' at two very different things.....
Could you put some logging code at various stages of the startup to give some indication of how far the process gets (maybe adding timing info so that if the application does eventually start you will have a better idea of where the delay occurs).....