Csrf Attack On Checkout

Good afternoon, hoping someone can help me please. I have a lot of customers message me about receiving this error: 'Error: Possible CSRF attack'.

They receive this when trying to checkout, viewing the basket or finalising the order. This seems to happen more so when they have multiple products from multiple vendors.

I have previously increased my max_post_size & max_input_vars as I had the CSRF attack issue pop up on the admin panel when trying to make multiple changes. This problem is now fixed though the CSRF attack occurring on checkout is still happening. Ive looked and can't find a fix for this.

This is costing me a lot of sales at the moment so any help would be great!

The issue seems difficult to constantly replicate so I cant find a cause or effect. My website is www.humblemarket.co.uk - if anyone would like to try and replicate.

Im not terribly brilliant with the server side of my store though my hosting company is easily accessible (hosted by site ground) so will be able to assist if anyone can recommend anything.

Thanks in advance