Placing echo session_save_path(); just before the @session_destroy(); line in access/logout.php and checking the html source of the logged out page will show where the session files are stored - these too must be purged periodically - they will generally be beyond the webroot and possibly out of reach of the php's permissions to destroy them and hence the error. In linux systems I have not found this issue.

[31-Jul-2017 13:53:27] 0::logout.php:50:[before upgrade] session_destroy() [<a href='function.session-destroy'>function.session-destroy</a>]: Session object destruction failed

The following links have more info on it:

https://stackoverflow.com/questions/8549757/why-session-object-destruction-failed
https://stackoverflow.com/questions/3538513/detect-if-php-session-exists
https://github.com/yiisoft/yii2/commit/457abe8939b0d1b7b41c33c4c335d7d6b8ae83a6