Orders with empty product list

[RabbitZzZ]

Hi,

on PS 1.6.1.10 we recently had the problem that orders in the backoffice appeared that had a fullfilled Payment (PayPal Plus) but showed an empty or incomplete product list.

Scrolling through the error logs I realized that a lot of times I get

PHP Warning:  Invalid argument supplied for foreach() in .../classes/Cart.php on line 3271
PHP Warning:  Invalid argument supplied for foreach() in .../classes/Cart.php on line 3291

as well as in other scripts that use foreach loops with the products in the cart. Could there be a connection? Why are there so many of these warnings in the first time? 
Besides that I couldn't find suspicious error messages at the times of the orders.

PHP 7.0.33

[RabbitZzZ]

[06-Mar-2019 15:51:09 Europe/Berlin] PHP Warning:  Illegal offset type in .../tools/swift/classes/Swift/Mime/SimpleMessage.php on line 404
[06-Mar-2019 15:51:10 Europe/Berlin] PHP Warning:  Illegal offset type in .../tools/swift/classes/Swift/Mime/SimpleMessage.php on line 404
[06-Mar-2019 15:51:11 Europe/Berlin] PHP Warning:  Invalid argument supplied for foreach() in .../classes/Carrier.php on line 1385
[06-Mar-2019 15:51:11 Europe/Berlin] PHP Warning:  Invalid argument supplied for foreach() in .../classes/Carrier.php on line 1385
[06-Mar-2019 15:51:11 Europe/Berlin] PHP Warning:  Invalid argument supplied for foreach() in .../classes/Carrier.php on line 1385
[06-Mar-2019 15:51:11 Europe/Berlin] PHP Fatal error:  Uncaught Error: Call to a member function displayPriceSmarty() on string in .../cache/smarty/compile/5d/2a/09/5d2a0978c35cb726b95eb4ed2e2b34676cd270fa.file.shopping-cart-product-line.tpl.php:129
Stack trace:
#0 .../tools/smarty/sysplugins/smarty_internal_templatebase.php(188): content_5c7fde50531549_82150981(Object(Smarty_Custom_Template))
#1 .../classes/SmartyCustom.php(317): Smarty_Internal_TemplateBase->fetch(Object(Smarty_Custom_Template), NULL, NULL, NULL, false, false, true)
#2 .../tools/smarty/sysplugins/smarty_internal_template.php(303): Smarty_Custom_Template->fetch(NULL, NULL, NULL, NULL, false, false, true)
#3 .../tools/smarty/sysplugins/smarty_internal_templatebase.php(157) : eval()'d code(331): Smarty_Internal_Template->getSubTemplate('...', NULL, NULL, 0, NULL, Array, 0)
#4 ... in .../cache/smarty/compile/5d/2a/09/5d2a0978c35cb726b95eb4ed2e2b34676cd270fa.file.shopping-cart-product-line.tpl.php on line 129

details from the PHP Errors

Edited March 6, 2019 by RabbitZzZ (see edit history)

[RabbitZzZ]

It seems like the error is happening, when a customer has the last available item of a product in his order.