Have you tried changing ExecuteURL to Redirect in the responseMode attribute?
<httpErrors existingResponse="Replace" defaultResponseMode="Redirect" errorMode="Custom">
<error statusCode="404" responseMode="Redirect" path="/index1.htm"/>...
I think creating a scheduled task for is a messy solution.
The reason is the PHP worker process does not have sufficient permission to delete the expired session files in that directory.
Open your php.ini file and change the session.save_path value to another directory other than C:\Windows\Temp (e.g. C:\PHP\Temp) with the right write and delete permissions ...
If you're like me on a shared hosting platform, without access to IIS manager, you can also edit the web.config file and manually add to the system.webServer element:
<httpErrors errorMode="Detailed" />
Afterwards I used my host control panel to recycle application pool.