Cannot unserialize ci_sessions from database

Another potential reason:

You can sometimes come across this issue if you are using different versions of PHP, or if you change the version of PHP you are using while a session was open.

For example if you have a session cookie with an app that uses PHP 5.6.* and then you try to use it with an app (that resides on another sub-domain) that uses PHP 7.2.*, then you are going to get a warning error. Or, if you had an open session and then you changed the version of PHP that you are using with your app (say if you are developing locally and switching around PHP versions), then you'll get the warning. So best to use serialize/unserialize and with PHP version that does not change.