Some customers can get past "The product quantity has been updated"

[schumi1971]

Some of my customers can get past "The product quantity has been updated" during checkout, nothing happens when they try to confirm the order.
I believe that those who have problems with this, are sitting on networks that have strict firewall rules.
The problems started probably after upgrading to VM 2.6.8 in July, because there was no one who had similar problems before the upgrade.
The same customers who are experiencing these problems, must also always click that they have "approve sales condition". Although they try to click this for good in the account maintenance, VM will not remember this for these customers.

Has anyone experienced similar or is there something I can try for the customers sitting behind these strict firewalls will have a normal experience of VM.
I appreciate all suggestions, I have tried to search the forum for others with similar problems, but found nothing