logged out when form get submitted..... Strange problem

Question

I am facing strange problem. I have form which has lots of hidden field and text field and one file field. And there are total six fields for selecting tagging on file which is to be uploaded.

When user select less parameter or less tags then form get submitted properly. but when user select more parameters in all six fields and submit form user get logged out of the application....

And one more thing i have increases maximum post size and maximum upload size.

This application is based on zend framework, Apache server, MySQL. Thanks in advance.......