Finding cause for ASP Error - iis

Question

I have a simple bit of code on my classic ASP website to send me an email when a page errors, as I have this set up in my web.config to point at a page to handle errors:
<httpErrors errorMode="Custom" defaultResponseMode="ExecuteURL">
<remove statusCode="500" subStatusCode="100" />
<remove statusCode="500" subStatusCode="-1" />
<remove statusCode="404" subStatusCode="-1" />
<error statusCode="404" path="/error_404.asp" responseMode="ExecuteURL" />
<error statusCode="500" prefixLanguageFilePath="" path="/error_500.asp" responseMode="ExecuteURL" />
<error statusCode="500" subStatusCode="100" path="/error_500.asp" responseMode="ExecuteURL" />
</httpErrors>
This is my simple bit of code:
...
set objError = Server.getLastError()
strNumber = objError.AspCode
strSource = objError.Category
strDesc = newstr(objError.Description)
strCode = newstr(objError.Source)
strLine = ObjError.Line
strASPDesc = ObjError.ASPDescription
strRemoteAddr = Request.ServerVariables("REMOTE_ADDR")
ref = request.servervariables("HTTP_REFERER")
str = request.servervariables("QUERY_STRING")
cookies = request.servervariables("HTTP_COOKIE")
ip_url = strRemoteAddr
ua = newstr(request.servervariables("HTTP_USER_AGENT"))
totalstring = objError.File & "?" & str
I then concatenate those values together and email the details to myself.
For an error, I'd get something like this:
strNumber: ASP 0126
strSource: Active Server Pages
strDesc: Include file not found
strCode:
strLine: 14
strASPDesc: The include file '/news/aw.asp' was not found.
strRemoteAddr: 2.101.166.175
ref: http://example.com/sites/support/
str: page=cd
cookies: usr1=62105233D8E2A062A55; fantastic%5Fcheese=1; _ga=GA1.3.1357551757.1476543431; __atuvc=2%7C41%2C1%7C42%2C0%7C43%2C0%7C44%2C9%7C45; __utma=154830755.1357551757.1476543431.1476822981.1479368067.3; __utmz=154830755.1476818175.1.1.utmcsr=(direct)|utmccn=(direct)|utmcmd=(none); ASPSESSIONIDQCBABQAR=KHGBAMBCCCENHCIPPCOFBJOO; __utmb=154830755.1.10.1479368067; __utmc=154830755; __utmt=1
ip_url: 84.92.46.118
ua: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:49.0) Gecko/20100101 Firefox/49.0
totalstring: /sites/support/page.asp?landscape=cd
However, I often only see this error:
strNumber:
strSource:
strDesc:
strCode:
strLine: 0
strASPDesc:
strRemoteAddr: 43.247.156.6
ref: http:/example.com/content/this.asp
str: 500;http://jimpix.co.uk:80/words/check-username.asp
cookies: ASPSESSIONIDSAABARBQ=ACDDOHOCEKIEFOCKFBFIHJBC; _gat=1; __atuvc=129%7C46; __atuvs=582e898682f62926059; _ga=GA1.3.491207786.1479439414
ip_url: 43.247.156.6
ua: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/54.0.2840.99 Safari/537.36
totalstring: ?500;http:/example.com:80/content/this.asp
There are no error details as such, but it's always on the same page. I can't work out what's causing it.
Is there any more detailed error logging I can do to find out more?

Answer 1

Server.getLastError() only remains populated until the response buffer has started to flush content to the client, reasons for this could be:
response.buffer is set to false
response.flush has been called
response.end has been called
There is a reference to this in the documentation:
This method is available only before the .asp file has sent any
content to the client.
This is the only thing i can think of that would explain why your Server.getLastError() seems to be blank.