Maybe this is a hint ?
to get auto log-in there are settings in the board.
just cause you tell your browser to save your password does not mean you will be auto logged in every time. all this does is save the password in your browser so you dont have to type it every time.
make sure to check the
acp/security settings
and make sure you have
Allow persistent logins:= Yes
Determines whether users can autologin when they visit the board.
Persistent login key expiration length (in days): = 0
Number of days after which persistent login keys are removed or zero to disable.
Validate browser: = Yes
Enables browser validation for each session improving security.
if all these are turned on and you still have problems then it is a security setting in/on your computer
https://www.phpbb.com/community/viewtopic.php?t=2148309