POST data into ASPX with requests. How to fix "remote mashine error" message - asp.net

Question

I'm trying to scrap data from aspx page using request with POST data.
On parsed html I'm getting an error "An application error occurred on the server. The current custom error settings for this application prevent the details of the application error from being viewed remotely (for security reasons). It could, however, be viewed by browsers running on the local server machine."
I was searching for solutions a while but frankly I'm new in Python and can't really figure out what's wrong.
The ASPX has javaonclick function which opens a new window with data in html.
The code I've created is below.
Any help or suggestions would be greatly welcomed. Thank you!
import requests
from bs4 import BeautifulSoup
session = requests.Session()
url = 'http://ws1.osfi-bsif.gc.ca/WebApps/FINDAT/Insurance.aspx?T=0&LANG=E'
r=session.get(url)
soup = BeautifulSoup(r.content,'lxml')
viewstate = soup.select("#__VIEWSTATE")[0]['value']
eventvalidation = soup.select("#__EVENTVALIDATION")[0]['value']
payload = {
r'__EVENTTARGET': r'',
r'__EVENTARGUMENT': r'',
r'__LASTFOCUS': r'',
r'__VIEWSTATE': viewstate,
r'__VIEWSTATEGENERATOR': r'B2E4460D',
r'__EVENTVALIDATION': eventvalidation,
r'InsuranceWebPartManager$gwpinsuranceControl$insuranceControl$institutionType': r'radioButton1',
r'InsuranceWebPartManager$gwpinsuranceControl$insuranceControl$institutionDropDownList': r'F018',
r'InsuranceWebPartManager$gwpinsuranceControl$insuranceControl$reportTemplateDropDownList': r'C_LIFE-1',
r'InsuranceWebPartManager$gwpinsuranceControl$insuranceControl$reportDateDropDownList': r'3+-+2015',
r'InsuranceWebPartManager$gwpinsuranceControl$insuranceControl$submitButton': r'Submit'
}
HEADER = {
"Content-Type":"application/x-www-form-urlencoded",
"Content-Length":"11759",
"Host":"ws1.osfi-bsif.gc.ca",
"User-Agent":"Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/49.0.2623.87 Safari/537.36",
"Accept":"text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8",
"Accept-Language":"en-US,en;q=0.5",
"Cache-Control": "max-age=0",
"Accept-Encoding":"gzip, deflate",
"Connection":"keep-alive",
}
df = session.post(url, data=payload, headers=HEADER)
print df.text