(I moved some replies from the other thread here, as that thread has to do with the worker iframe hanging, as when the code goes into an infinite loop, or you have a tab open with a debugger paused.)
The 403 error suggests that this is an authentication problem, as with the other “Oops unable to connect” messages.
Fil, is there something that is causing your cookies to expire (more frequently than expected) in your browser? For example, are you manually clearing your cookies, or do you have an extension or browser setting that is automatically flushing cookies?
If you clear cookies but still have Observable open (say in a background tab), that would lead to the “Oops unable to connect” message when you return to the tab without reloading. Whenever your cookies are cleared, you’ll have to reload each open Observable tab before the tab will allow you to perform any authenticated action.