Using Obsidian Sync behind a proxy which doesn’t allow WebSocket connections, for example McAfee Web Gateway.
Use HTTP polling when syncing fails via WebSocket.
There are WebSocket libraries which support this fallback option.
Alternatively, maybe, syncing could be done in the main process (Node.js), so that it bypasses the operating system proxy settings. But I’m guessing this would be more work and harder to test.
For some users, the *.obsidian.md domain could be added to the proxy server’s ignore list - In my case, it’s not possible.
When the proxy is imposed via corporate policy, the proxy cannot be disabled. Even --no-proxy-server chromium/electron command line switch will not work as the corporate policy will take priority (source: 247461 - chromium - An open-source project to help move the web forward. - Monorail )
The only workaround I can think of to get sync working would be to run Obsidian inside a VM, but that would be quite difficult and the users might not be allowed to install a VM on their work laptop.