PHP Errors in "Text Area" fields, Causes Ajax Error & site does not proceed

I have been having this error for months and now i am able to reproduce it, its running down my shop & i need HELP. ( i am seeking your assistance).

1. While installing Ubercart i made this change (This change enables a client to post more information in the product attribute check boxes)

MODIFICATION: /sites/all/modules/ubercart/uc_attribute/uc_attribute.module
In that document I replace all "textfield" to "textarea". This document should have 3 of them. READ: Solution for text area support (https://www.drupal.org/node/496450)

2. Once i do this, visit my site https://xx.net/order

3. The modified fields include (Paper description, Paper title & Name/email).

4. Download a sample file i have attached (.pdf) below its called "Test.pdf". ONLY use this sample pdf for testing, some content work some dont.

5. Now copy and paste the contents of that pdf in to the following modified field (Paper description or Paper title on this page https://xx.net/order). Also fill the "required fields"

6. Immediately you do so, Ajax throws two errors, and it does not paste the contents. The customer is NOT able to proceed to payment section

Below are the two errors (thrown in recent log)
1. "Invalid form POST data."
2. "Notice: Undefined index: form_build_id in ajax_get_form() (line 325 of /home/myesdowz/public_html/includes/ajax.inc)."

This only happens when customers try to copy "SOME" content in to the text area sections. What is hapenning here?. This thing is making me lose so many clients, (This fields have to be textarea & not text field because clients post very long information at times)
Help Me solve this before i close SHOP Please.

I have done a fresh install the same thing happens across all my sites. HELP