Can’t access Dashboard & Updates pages of admin site

Hi all,
Having some trouble with a WordPress site which has been running fine to-date.

Attempted to login to admin section to make changes and could not login. Got HTTP error 500.

Having looked online, managed to do a couple of things, namely:
Increased the php_memory limit
Disabled all plugins – via cPanel file manager

I can now access the admin site, albeit occasional errors. However, I cannot access the Dashboard or Updates pages at all. There is 1 update showing, which appears to be a Translation Update but whenever I click on Dashboard or Updates, I get the HTTP ERROR 500.

I have got error logging enabled, which shows the following error continuously:

PHP Fatal error: Class ‘Requests_Response’ not found in /home/aimd/public_html/eccytennis2/wp-includes/class-requests.php on line 629

Line 628 – 632 of that file is:

protected static function parse_response($headers, $url, $req_headers, $req_data, $options) {
		$return = new Requests_Response();
		if (!$options['blocking']) {
			return $return;
		}

Does anyone have any ideas?

Thanks,

Atif