I'm seeing instances of this in the log directory, although I don't yet know if that is being generated at the same time as the Ajax timeout occurs
PHP Notice: Malformed value for session-based shipping module; customer will need to re-select: false in /home/mysite/public_html/includes/classes/order.php on line 323
I recall reading that others have experienced this error log and it may be a ZC bug?

Also seeing a few instances of this:

[26-Apr-2021 16:44:22 Europe/London] Request URI: /ajax.php?act=ajaxOnePageCheckout&method=updateShipping, IP address: 79.66.5.148
#1 trigger_error() called at [/home/mysite/public_html/includes/classes/OnePageCheckout.php:1361]
#2 OnePageCheckout->inputPreCheck() called at [/home/mysite/public_html/includes/classes/OnePageCheckout.php:903]
#3 OnePageCheckout->validateBilltoSendto() called at [/home/mysite/public_html/includes/classes/ajax/zcAjaxOnePageCheckout.php:61]
#4 zcAjaxOnePageCheckout->updateShipping() called at [/home/mysite/public_html/ajax.php:85]
--> PHP Fatal error: Invalid request, tempAddressValues not set. in /home/mysite/public_html/includes/classes/OnePageCheckout.php on line 1361.

And this:
[31-May-2021 12:14:02 UTC] PHP Fatal error: Uncaught Error: Call to a member function isLoggedIn() on null in /home/mysite/public_html/includes/classes/observers/class.checkout_one_observer.php:341
Stack trace:
#0 /home/mysite/public_html/includes/classes/class.base.php(118): checkout_one_observer->update(Object(notifier), 'NOTIFY_ZEN_IS_L...', '', false, NULL, NULL, NULL, NULL, NULL, NULL, NULL)
#1 /home/mysite/public_html/includes/functions/functions_customers.php(275): base->notify('NOTIFY_ZEN_IS_L...', '', false)
#2 /home/mysite/public_html/includes/templates/jsweb/common/html_header.php(156): zen_is_logged_in()
#3 /home/mysite/public_html/index.php(42): require('/home/balloonm/...')
#4 {main}
thrown in /home/mysite/public_html/includes/classes/observers/class.checkout_one_observer.php on line 341