I'm having this problem too, for the last week+. So, I have tried all the tricks; firewall, antivirus, etc. I even tried logging in on Lumyia from my phone. I was able to get through on the mobile data, but not the wifi.
I called my internet provider on two separate occassions, and nothing is being blocked. They even looked into anything that would block SL on their side.
I am waiting to hear back on my SL support ticket (that process takes forever) on whether my IP Address is being blocked.
Needless to say, extremely frustrating. If anyone has learned what the cause it, I would love to hear it... At this point I'm curious if I'll ever be able to get back in to SL.