I also cannot repro logged out on Firefox Mobile (Android, of course) with the identical details as yours, navigating directly to the home page as well as to other pages and back, either via site navigation or forward/back.
I managed to get a curl command that reliably creates the “Oops” error on the home page.
It seems to have something to do with my login cookie. I can probably fix this by clearing cookies for discuss.python.org.
Before I clear my cookies and login again, are you interested in the curl command?
If so, is there some way I can share that privately due to the login cookie?