It may help.
By re-reading the code I have an idea of what I could maybe change/check, but I won't have time before Monday. Anyway the issue would be caused by the switch between http and https between two visits with a timeout of the session, so if you always use the same version it should work as before. The switch from one to the other may be caused by following links on the forum or using bookmarks that point to one or the other. But while you are on the site normally all internal links point to the same version as the one you already use (especially since the fix I did a few days ago).

Maybe I should solve the issue by simply forcing to use https version for login.
The reason why I don't force https on this site is that in case there is an issue with the certificate, the site can still be used in plain http.