The first time you launch EP (after an application recycling) - it does take a while for EP to load.  Sometimes, you might event see a SharePoint exception error but if you click on the refresh button in Internet explorer and wait couple more minutes then you will eventually see the EP page.   The exeption error is due to timeout issue.  It just takes a long time to launch EP the first time.  You can do the following to prevent the exception error due to timeout -

1.  Add HKEY_CURRENT_USER\SOFTWARE\Microsoft\Windows\CurrentVersion\Internet Settings\ReceiveTimeout (DWORD), and set to 4800000 (4800 seconds).   (this is per user's workstation)

2.  On each user's Internet explorer options, up the Temporary internet file disc space to user size to 250.  (tools>Internet options, under general tab click on the settings button under Browsing history) and also change "check for newer versions of stored pages" to Everytime I visit the web page.

3.  Add httpRuntime tag executionTimeout attribute to 900 (Seconds) in the WSS web.config (on your EP web application folder)
Your EP web.config will be on the IIS server under a directory similar to this - C:\inetpub\wwwroot\wss\VirtualDirectories\EPPortNumber
See this link for more information -

http://msdn.microsoft.com/en-us/library/e1f13641(v=vs.71).aspx

 

 

Written by:  Sunny Wong