Hey @realpolitik - welcome to the Community! And thanks for reaching out.
This is unfortunately something that has been surfaced a few times now, so it's definitely on our radar. It's been really challenging to consistently reproduce and thus identify a root cause, though, so any information you can share (such as HAR files) would be much appreciated!
At a high level, there seems to be a race condition in the initialization of the runtime environment that is exacerbated by specific client-side conditions - potentially OS or RAM/CPU resourcing. This thread identified Windows clients as being particularly susceptible, for example. Disabling Efficiency Mode might be a good place to start, as that has been a source of issues before.