Quite a few people have been reporting 403 errors. I put in a ticket to Squarespace and here's the response.
The typical reason for getting this error message is because your customers IP addresses are being blocked from our servers...
Although clearing the cache and deleting the cookies can help. There is one big step that your viewers can do to resolve this, and that is to have them go to a Squarespace 7 site like http://marquee-demo.squarespace.com/.
This site will prompt them to enter in a CAPTCHA and remove the block from their IP.
There are small cases where doing this wont work, and if you happen to run into one of these cases. Please have them take a screenshot of the error message, and get a copy of their IP address, and either write into us personally, or you can even forward us the information so that we can escalate the IP address, and get a deeper look into it.
Hope this helps.