this condition could also indicate a possible active attack on the TCP link

Hi there, I keep seeing this  (this condition could also indicate a possible active attack on the TCP link) in the ssl vpn log, always from http://184.105.247.196/ 

I have enabled PPTP and its working, fine.  Should I do anything? or is this to be expected?

The UTM is connected via a Vigor router with a public IP alias so NATd and not really fully visible

Thankyou