Modified mysite/code/Page.php with following code:

function getCMSFields() {

$rhs_sidebar_content = new TextAreaField('rhs_sidebar_content', 'RHS Sidebar Content');
$rhs_sidebar_flag = new CheckboxField('rhs_sidebar_flag', 'Show RHS Sidebar');
$lhs_sidebar_content = new TextAreaField('lhs_sidebar_content', 'LHS Sidebar Content');
$lhs_sidebar_flag = new CheckboxField('lhs_sidebar_flag', 'Show LHS Sidebar');

$fields = parent::getCMSFields();

$fields->addFieldToTab('Root.Content.Main', $rhs_sidebar_content, 'Content');
$fields->addFieldToTab('Root.Content.Main', $rhs_sidebar_flag, 'Content');
$fields->addFieldToTab('Root.Content.Main', $lhs_sidebar_content, 'Content');
$fields->addFieldToTab('Root.Content.Main', $lhs_sidebar_flag, 'Content');
   
return $fields;
}

Get error in admin of:- Fatal error: Class 'TextAreaField' not found in C:\wamp\www\ss\mysite\code\Page.php on line 15

Fault is intermittent but occurs mainly when clicking from "Site Content" to any page on the LHS menu

Fault can be worked around by adding line:

include_once('forms/TextAreaField.php');

at top of function

Suggests that manifest builder intermittently fails to pick up TextAreaField.php

some info:
- a silverstripe PHP-class should always be contained in a file with the same name (e.g. "class TextareaField" should be contained in TextareaField.php) - this is how the manifestbuilder magically detects the required includes
- PHP-classnames are case-sensitive
- filenames might be case-sensitive depending on the operating system

for your specific problem: the class is actually called "TextareaField" with a lowercase "area"
i've closed the bug as "invalid" - cheers for your bugreport!