I was trying to log into SEDE using the Stack Exchange OpenID function. However, without any erros, I kept not being logged in.
Eventually, I discovered I was using the HTTP to connect to SEDE, not HTTPS. After switching to HTTPS, the login worked.
Can you either fix this, move SEDE to HTTPS altogether, or at least show a descriptive error (perhaps block the login page on HTTP with an error?)
Browser: FF 60.0.2 x64 on Win 10.
Related: Is SEDE secure?
Edit: steps to reproduce:
On a clean Firefox (safe mode + private browsing), first, go to https://data.stackexchange.com. Log in succesfully. Now, open up a new tab to http://data.stackexchange.com. Try to log in. Doesn't work.