As Vladimir said, without SSL inspection, it is an expected behavior in certain case.
Other than that, I did has a similar situation on my side and I ended up getting a custom fix from TAC becore the apache services responsible for showing the block message was basically "out of port" of overloaded.
It could be a similar situation on your side depending of the number of user you have. In my case, we had about 2K user total.
I suggest to you to open a case at TAC about this issue. Other than that, if you want, I may be able to find more information regarding the custom patch I was running in R80.10 regarding this issue.