500 error when processing payment by PayPal Express Plugin

[Ms_L]

I'm using PrestaShop 1.6.0.9, all modules are up to date, no third party module, default theme, default PayPal UK module v3.11.6

Since updated all modules on 8 August 2018, PayPal Express plugin returns 500 server error after the final step i.e. confirm payment is clicked i.e. payment is submitted.

• I've tried PayPal Technical Support many times and they checked and said there is no error on the PayPal side and that it is a Shopping Cart error. 
• I've checked with my hosting Technical Support many times too and they confirmed that there is no error logged on the server and so it is not a server error.
• I've tried changing the php version 5.4, 5.5, 5.6, 7 and 7.1 and it did not solve the issue.
• I've also tried the approach mentioned on https://www.prestashop.com/forums/topic/607502-paypal-express-checkout-leads-back-to-my-website-500-server-error/?tab=comments#comment-2943922

None of the above worked for me.  I've run the Error Reporting and it returns the following:

"Fatal error: Uncaught exception 'PrestaShopException' with message 'Property Address->phone_mobile is empty' in /home/abcde123/public_html/classes/ObjectModel.php:846 Stack trace: #0 /home/abcde123/public_html/classes/ObjectModel.php(260): ObjectModelCore->validateFields() #1 /home/abcde123/public_html/classes/ObjectModel.php(464): ObjectModelCore->getFields() #2 /home/abcde123/public_html/classes/Address.php(170): ObjectModelCore->add(true, false) #3 /home/abcde123/public_html/modules/paypal/express_checkout/payment.php(265): AddressCore->add() #4 {main} thrown in /home/abcde123/public_html/classes/ObjectModel.php on line 846"

 

If you have had similar issue and solution, please comment and share your solution.  Greatly appreciate your help.

Edited October 16, 2018 by Ms_L (see edit history)