I have had the same problem since week ago (haven't tried before that). On Safari Fastmail works.
All requests are 200 OK (there are 2 fewer requests compared to Firefox).
No JS errors in console - I don't see Content Security Policy-related related errors neither compatibility mode enabled or disabled.
If someone has suggestions on what to try either to resolve the issue, or get more information, I'm happy to do some tinkering!
Orion, OS version; hardware type:
MacBook Pro (14-inch, 2021)
macOs Monterey 12.5.1 (21G83)
Version 0.99.119-beta (WebKit 615.1.1)