On my production site, if you try to login to the account from the sign in page it fails the first try. There is no error message, it just empties the form and doesn’t log you in. If you try a second time it works and logs the user in.
Now, if you try to login from the onepage checkout it empties your cart and doesn’t log you in and redirects you to the empty cart page. If you then add an item to your cart and go back to checkout and login again it logs you in and merges your cart with the item(s) it previously cleared out.
If you log out and try to log back in it works on subsequent log in attempts. It's just the very first time trying to login to an account that fails with no visual indication as to why.
I've verified that the form key is on the page for both login locations. My cookie domain is set as .MyUrlHere.com
Running Magento v. 22.214.171.124