I'm having the same problem and I suspect our university's firewall. (Zotero 7 continues to work very well on my private laptop.)

Question: Zotero 6 used to be able to synchronize on my work computer as well. With the upgrade to Zotero 7, sync no longer works. Any chance to get it working by e.g. using a different mechanism than websocket?

edit: https://echo.websocket.org/.ws works from the university computer, and I can also visit www.zotero.org so it's not the domain itself that is blocked

I solved it :-)

When I was unable to submit the debug log that you requested, I wondered if there was some general connection problem. I normally use chromium-based browsers on my work computer, but now I tried to load a web page with the Firefox version that is provided by our institution. This failed! Some trial and error later I learned that I had to set the proxy settings in Firefox from "system" to "auto detect".

In Zotero's advanced preferences editor, I changed "network.proxy.type" from the default value 5 (use system proxy settings) to 4 (auto-detect proxy settings) and then, finally, Zotero sync worked again. :-)

https://kb.mozillazine.org/Network.proxy.type

Dan, let me sincerely thank you for your relentless efforts and work for Zotero. This has been my daily workhorse since ~2007, when Zotero was still very young :-) Thanks a million to you and your entire team.