TYPO3 Core

I am experiencing an issue where sometimes I am not able to login to backend, a 4.7 site was upgraded/redone to 6.2 then to latest 7.6.9, have recreated passwords for all users and can often log in without problems first time, but sometimes I am not able to login even for a few hours.

When I check logs I can see "LOGIN ATTEMPT Login-attempt from .*..* () for username 'admin' with an empty password!" so it seems like even though I manually enter the password each time somehow the system get's an empty password request. Very inconsistent unfortunately. I have tried using different browsers, sometimes it helps, not always.

Possible causes:

1) Core bug - Have found https://forum.typo3.org/index.php/t/203974/ but no solution there.
2) Server related issue
3) Browser / Password Manager related - typing password in manually and submitting decreases the number of failed logins but it does not work always.
4) I have noticed that sometimes blank duplicate sessions exist in TYPO3 admin / Backend Users / Online users, also for some users it's showing that they are logged in but they are actually not - possibly related to login issue?

Any thoughts are welcome. Thank you.