This simple thing has really caused me a great deal of trouble.
I had an account created at time of migration, and used it for a time with no issues.
Then, I started having problems where I would be asked to Sign In when I try to post. I would do so and continue.
Then. I found that when asked to Sign In and trying to do so, all I would get is a screen refresh with the 'Sign In' button remaining at that instead of changing to Sign Out.
I eventually gave up and deleted my account - I had better thing to do than play with a badly made web site.
After a few months, I thought I would try again. As I had deleted my old account I created another one - despite this web site telling me someone else had my email address (of course it was just me that had it - this web site just hadn't deleted my details when I deleted my account - isn't there something devious about that?)
Anyway I continued to create a new account with a different email address and managed to get in. All seemed fine, logging in seemed fine, posting was fine. OK, all this lasted about a day. Then I started getting the 'please Sign In before posting' again, and sure enough, the drop down showed the Sign In button.
A bit mixed up here as sometimes, I would get the 'Please Sign In ...' sometimes just a screen refresh.
Laterly, and right now as I type this, I have just made a new post successully, and am making this one and the drop down is currently showing 'SIGN IN.
So, all along, this site has been lying to me. I would constantly try and Sign In when in reality, most of the time, I was actually already Signed In!
Funny how millions of other web sites can manage to Sign In members without issue ..........