HTTP 500 errors after recycle

Question

I've got 10 servers with IIS set up identically on each, with two active applications. Occasionally, immediately after our 2am IIS recycle (configured through the recycling properties page) one of the two applications on a random server will start throwing HTTP 500 errors. The other application will continue serving requests just fine.
What I've just worked out though is that it's only one certain request that get the 500 errors. These requests are coming from our load balancer's monitors every 5 seconds from each of 4 nodes. However, changing the capitalisation on the requests will make them succeed, ie /<stuff>/appserver.asmx fails, but if I change it to /<stuff>/AppServer.asmx, or /<stuff>/appsErvEr.asmx, these will succeed, but the original keeps failing. Regardless of whether it's being sent from the LB or my local machine.
An IISReset fixes the issue every time. Nothing in the httperr logs, event viewer just gives a generic "An unhandled exception has occurred" message. I thought it might have been something to do with overlapping recycling so I set it to TRUE on half of them and FALSE on the others but the error still occurred on both sets of servers. Recycling is definitely happening as I can see both worker processes change their PIDs at 2am. Any help appreciated. Thanks.

Answer 1

Hey @Sam Wu-MSFT . I can't see anything in the FRT logs that gives me any more idea of why this is happening. Here's the lines before the error. Yesterday I scheduled through Task Schedular an iisreset across all my servers after the 2am recycle, and ALL of them started throwing the error. But running an iisreset now doesn't replicate the issue. Weird.

87. NOTIFY_MODULE_END ModuleName="HttpRedirectionModule", Notification="MAP_REQUEST_HANDLER", fIsPostNotificationEvent="false", NotificationStatus="NOTIFICATION_CONTINUE" 00:34:00.356   
88. NOTIFY_MODULE_START ModuleName="ManagedPipelineHandler", Notification="MAP_REQUEST_HANDLER", fIsPostNotification="false" 00:34:00.356   
89. AspNetMapHandlerEnter  00:34:00.356   
90. AspNetWebEventRaiseStart Data1="System.Web.Management.WebRequestErrorEvent", Data2="3005", Data3="0" 00:34:00.356   
91. AspNetWebEventRaiseEnd  00:34:00.356   
92. GENERAL_SET_RESPONSE_HEADER HeaderName="Cache-Control", HeaderValue="private", Replace="true" 00:34:00.356   
93. GENERAL_SET_RESPONSE_HEADER HeaderName="X-AspNet-Version", HeaderValue="2.0.50727", Replace="true" 00:34:00.356   
94. MODULE_SET_RESPONSE_ERROR_STATUS  
  
Warning  
 ModuleName="ManagedPipelineHandler", Notification="MAP_REQUEST_HANDLER", HttpStatus="500", HttpReason="Internal Server Error", HttpSubStatus="0", ErrorCode="The operation completed successfully.  
 (0x0)", ConfigExceptionInfo="" 00:34:00.356   
95. GENERAL_SET_RESPONSE_HEADER HeaderName="Content-Type", HeaderValue="text/html; charset=utf-8", Replace="true" 00:34:00.356