Customers on imac using and safari and firefox can't place orders on my website

Customers on imac using and safari and firefox can’t place orders on my website. This is a typical customer report,
‘There appears to be a problem between you and PayPal. I have tried to order a couple of sponges this morning and get as far as PayPal sending me back to your site which then says everything is invalid.’

I think it might be the theme becuase this customer said he had been able to order before I changed the theme. The fault is triggered when the customer tries to pay - therefore they think it is paypal, but I would like to find out if it is the theme and how to fix it. I hope somebody can help me.