In the error log provided, I see 12 references to one account - ais2234.
But NOT bunches and bunches of accounts with issues or that would be a very
large error_log. Did you do through the error_log and record what accounts?
And after that did you fix manually?
You say you fix those manually. Does that user have anymore issues after you fix manually?
"but still do not know the error is it a bug in moodle??'
What if I said yes, it's a bug (even though looking through Tracker,
I can't find anything that matches. But, maybe you will have more luck:
https://www.google.com/search?q=moodle+tracker+invalid+login
https://www.google.com/search?q=moodle+tracker+password+reset+loop
"Secondly, most of the error seems from wrong user id??"
And you were the one that used csv to create accounts, right?
Neither you or I can control what users do when they attempt to login.
Did you look at the listing of users in the site?
"stil with the shared hosting"
This could be the issue. But I would have no way of knowing.
"Also some users faced with password reset loop and the new password is stored but still ask them to reset there pass"
You know their user id's. Do they show in the new error_log?
You just switched from DB to files for sessions.
That won't fix issues that exist with accounts for whatever reason.
Asked you to check on the security for accounts setting in config of Moodle - how many caps for accounts, how many non-alphanumeric characters, etc. etc. to see if there isn't some sort of issue with that not matching account creation via csv or them having issues when trying to login .... Now don't ask me where that is ... use Admin Menu Search box.
Did you?
?????
'spirit of sharing', Ken