First thing that would be useful to know is whether they are happening on a freshly used browser instance / tab, or if they’re only happening if the browser/tab has been left idle for a while.
I get them on the main community sometimes as well, but it’s always when I’ve left a tab open but unused for a significant period of time (eg with a half-written reply and then had to go join a Zoom call before coming back an hour later, finishing the draft, submitting it and getting the error). What happens in these instances is that your session with Flarum goes stale and needs a refresh to kick it back into action (either a page refresh, or loading a new page).
