I manage over a dozen Squarespace sites so the bulk of my time is spent in Squarespace's config mode. I too am experiencing this and have been since the latest Safari update before the holidays. It happens with all Squarespace sites and is very frustrating. I'm having to switch to Chrome to get work done.
I have reached out to Squarespace support and they did not respond. There is a forum post on their support site which was closed with the excuse that the problem was not reproducible.
I actually just got my 1-year-old iMac 5K back from Apple from having a bad RAM chip and had to do a clean install of the OS since it caused me to corrupt it when I was using Disk Utility to troubleshoot. And I did a clean install - not a restore from Time Machine backup. So this is not a caching issue.
I still need to try and see if Ghostery is causing the crash. I've disabled it in the past to no avail, but I wonder if it's installed if it causes the problem which persists even after disabling/uninstalling. The problem is I don't have time for troubleshooting...
I hope this info helps. I know it's not a solution. Just wanted to commiserate. 🙂