Magento – How to enable https for the magento site

I can't tell you what causes the problem you have because it seems to be quite localised. But here is the way to debug it.

The first line of your message leads us to Mage_Install_Model_Installer_Config::_checkUrl method. It sends request which is handled by Mage_Install_WizardController class.

Then the response body is checked in Zend_Http_Response::getBody if header contains transfer-encoding string and if it is chunked. If so Zend_Http_Response::decodeChunkedBody is called and if

! preg_match("/^([\da-fA-F]+)[^\r\n]*\r\n/sm", $body, $m)

The second line of your error is thrown.

So the first thing is to find out why do you have transfer-encoding=chunked and the second is why Mage_Install_Model_Installer_Config::_checkUrl is called at all as you said you disabled base url checks(s!).

UPDATE

Of course the easiest workaround would be to comment out the contents of Mage_Install_Model_Installer_Config::_checkUrl except return statement. Then you will be able to see if there will be more problems.

If you want to disable secure url for admin, you need to set the path web/secure/use_in_adminhtml value to 0 (or delete the record).