Just now, I got logged out twice when opening the first page of the
Test Beds thread, but only when there was a session id (&sid=...) in the URL. It didn't happen when opening the last page of the thread from a notification email, perhaps because there was no sid.
I got logged out by surprise the other day, too, though it wasn't repeated.
I use Firefox, but don't have an RSS reader extension in this profile. My extensions are Ublock Origin and Stylus. The latter I use on this site so I can have a good dark theme with a nicer shade of blue.
I wonder if it's anything to do with Firefox's privacy protection. It now has an option to "copy link without site tracking," implying it has code to recognize session ids. Maybe Firefox freaks on the removal of &e= with the addition of &sid=. Maybe something similar happens when a page is loaded from an RSS reader, but I wouldn't know.
...And I also got logged out when I hit Preview for this post. Maybe it was something to do with the testing I was doing on the above thread.