Marketpress 10525 Paypal error

Having a very odd issue with the Marketpress I've set up. Using the latest versions of WP,MP, Frame market etc.

A customer was trying to purchase an item on Monday night but told me they couldn't due to a Paypal error. I tried myself and found this to be true. While clicking through the checkout, when usually you would be directed to Paypal, you arrive at an unecessary page with just a single 'Continue Checkout' button. Click that and you then hit a new page containing the following error

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There was a problem connecting to PayPal to setup your purchase. Please try again.
10525 - Invalid Data - This transaction cannot be processed. The amount to be charged is zero.
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I spoke to @Timothy in chat about this who suggested removing AddThis. I did, and all of a sudden it was working again, in all browsers. Huzzah

Last night I received another email from a customer saying the site had Paypal issues. I checked and same error again.

Came to work this morning to fix the issue and the site was working fine - no error message.

Four hours later and the site is now displaying the error message again.

This is really frustrating and very damaging to the site which went public this week.

I found a similar thread https://premium.wpmudev.org/forums/topic/error-10525
but it leads nowhere.

Can anyone help with this issue as the website is getting a bad name before it's had a chance to get off the ground.