How to find cause of Classic ASP page errors - IIS8

Question

I'm running a Windows Server 2012 R12 running IIS8.

I have a classic ASP website running on it, and for the most part it runs fine. The website gets about 1.5 million page views per month (approx 400,000 unique page views).

I have a custom error page for 500.100 script errors, which stores details about the error in a database and sends me an email so I know if a page errors.

I have them set up via this in web.config:

<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>

On the error_500.asp the page includes this code:

On Error Resume Next
Response.Clear
Dim objError
Set objError = Server.GetLastError()

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

For some reason, I often get errors like this:

strNumber:
strSource:
strDesc:
strCode:
strLine: 0
strASPDesc:
strRemoteAddr: 98.168.145.166
ref: http://example.com/content/another-page.asp
str: 500;http://example.com/content/this-page.asp
cookies:    ASPSESSIONIDQQRTDDBQ=IBBOMHEDGEEGKAPPLDGLDHLE; _ga=GA1.3.1626071938.1497868025; _gid=GA1.3.629715385.1497868026; _gat=1; __gads=ID=c83d077246e96531:T=1497868028:S=ALNI_MY2v7GwftRZ5EMHIdi_5lEA8TDAgQ
ip_url: 98.168.145.166
ua: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/58.0.3029.110 Safari/537.36
totalstring: ?500;http://example.com/content/this-page.asp

So I can see the page is erroring, but there is no error message, or line number, or anything else.

I don't know how to find out more about these errors, and wondered if anyone might be able to advise please?

Sometimes I have had up to 10,000 of these all at the same time, which I presume is a hacking attempt, but I am not sure how I can run more detailed debugging etc?

I don't want to log everything on the site as it might fall over if I start doing that, possibly?

Any advice would be much appreciated, as I realise I need to do something to debug or log errors in more detail, but have no idea where to start.

Any advice much appreciated, thanks.