Hello,
By "test it", I think Mohamed means see if the issue for some of your customers disappear, of course that will take time to check this point.
Your HikaShop version is very old, and you have to know that right now we are on the 2.6.4, and lots of changes have been add, and especially on the checkout process.
Your issue isn't known by our team, and so it's just a guess but maybe some php function are deprecated or unused right now...
But without knowledge on how reproduce we can't be sure of it.
Other possible root issue, some custom code, directly in HikaShop files ? in override checkout view ?
Last idea, maybe a plugin is in conflict with HikaShop (recent plugin that works with an old HikaShop version ?...)
Hope this few idea can help you to solve this.
Regards