Get the list of recently modifed records

Get the list of recently modifed records

Hi,

I'm trying to get a list of recently modified records of Quotes and I getting this error:
code:"INTERNAL_ERROR"
details:""
message:"Internal Server Error",
status:"error"

Example VB.NET code :
Dim request As HttpWebRequest = WebRequest.Create(" https://www.zohoapis.com/crm/v2/quotes")

with request
.Headers.Add("Authorization", "Zoho-oauthtoken " & AccessToken)
.IfModifiedSince = "2018-03-01T12:00:00+01:00"
.Method = "GET"
end with

Using response As HttpWebResponse = CType(request.GetResponse, HttpWebResponse)

Using sr = New StreamReader(response.GetResponseStream())
jSON= sr.ReadToEnd()
End Using

End Using

if I don´t add header "If-Modified-Since" the code works perfectly

Thanks in advance.