I have encountered this issue as well.
I have as well. For those saying that it can't be an issue if you're posting, I think you're misunderstanding the issue.
Logging in functions, but immediately after login, when redirected back to where you were before logging in, the page doesn't refresh and instead loads from cache. It appears like you didn't log in. If you refresh the page, everything is fine. If you click to view another page, everything is fine (fine meaning it shows you logged in). It's only that initial redirect after logging in that seems to load the page from cache and make it appear like you're not logged in.