maincore.php on line 72

Did you delete the servers temp folder?, it must exist for sessions, temp upload files etc.
All sessions should auto destroy after 48h, but naturally it will grow with large amount of visitors.
Try logout and login again to recreate the session.

In which directory, you can reduce this limit?

You can't. It's an embedded security measure. For each form loaded, the field settings are secretly created on your server side. It contains all the rules for each field. Any non empty $_POST variable goes to compare and validate accordingly, and if just 1 is missing or tampered, the whole site will stop. That, is one of the primary feature of 9 - automatic sanitization. We're covering all possible input types - email, tel, name, address, url, date, etc. It all depends on $_SESSION must start.