it seems that https proxy is not stable. I have the same issue with a website and I explicity allowed it on the http proxy
seems strange
Code:
2009:06:27-20:42:19 Enterprise httpproxy[8978]: id="0002" severity="info" sys="SecureWeb" sub="http" name="web request blocked" action="block" method="GET" srcip="192.168.2.31" user="" statuscode="400" cached="0" profile="REF_TJkZFLrkmc (Zone 1 Proxy filter)" filteraction=" ()" size="2371" time="0 ms" request="0xa83d4490" url="http://update.filezilla-project.org/updatecheck.php?platform=i586-pc-mingw32msvc&version=3.2.5&osversion=5.1&beta=1" exceptions="" error=""