Guru's, since past 2 days we have tons (I literally mean TONS) of the following "General Exceptions" on our production environment, I don't understand what it is all about. thought I will shout out for help here, any guidance will be very useful. Please HELP:

AssemblyVersion: 04.08.00
PortalID: -1
PortalName:
UserID: -1
UserName:
ActiveTabID: -1
ActiveTabName:
RawURL: /DesktopModules/AuthenticationServices/DNN/ImageChallenge.captcha.aspx?captcha=A83C090573B41EE7579E4DB930362EBD9C1E33E3989D3470030912F4D3F2B9224FD07EAA6AD22A3340874D0CC6004B882E55CABE200FC6484BE747DC10E1B94102DE3AC4C1B3C6DB24847A115960BB16CD254110AE27EE8C1670D45480B386683D016B4B80F968FC4FDBE6D576C912AB697DBCC7F48D3926E137C9C427F13F46&alias=www.myproddomain.com
AbsoluteURL: /DesktopModules/AuthenticationServices/DNN/ImageChallenge.captcha.aspx
AbsoluteURLReferrer:
UserAgent: Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1)
DefaultDataProvider: DotNetNuke.Data.SqlDataProvider, DotNetNuke.SqlDataProvider
ExceptionGUID: 4e917651-246c-4cfb-a71e-80cf54d19e46
InnerException: Input string was not in a correct format.
FileName:
FileLineNumber: 0
FileColumnNumber: 0
Method: System.Number.StringToNumber
StackTrace:
Message: System.FormatException: Input string was not in a correct format. at System.Number.StringToNumber(String str, NumberStyles options, NumberBuffer& number, NumberFormatInfo info, Boolean parseDecimal) at System.Number.ParseInt32(String s, NumberStyles style, NumberFormatInfo info) at DotNetNuke.UI.WebControls.CaptchaControl.GenerateImage(String encryptedText)
Source:
Server Name: SINGLE32

Actually I never touched the web.config after we deployed it on prod. However I rechecked and it is exactly the same.

I had been monitoring it for almost 4 days now and I did not see the same exception, things are running nie and smooth. May be it was a hacking attempt? any thoughts?