Sandbox / Checkout error

I’m using PayFast Payment Gateway with the plugin and get the following error on checkout.

You’re in the sandbox. This is a testing environment used to ensure correct integration with PayFast. If unexpected, please contact the merchant you are trying to pay to resolve the matter.

Testing / Sandbox isn’t enabled. Clients processed many invoices successfully and all of a sudden they cannot checkout.

I disabled a couple of plugins to try to eliminate any conflict. Last time it was my cache plugin causing problems, but I couldn’t find anything.

I also don’t know if there was a recent update to the plugin, because I updated 15 this morning. I didn’t check which ones I updated, unfortunately.

Please help!