Guest users trying a coupon create error

I have a couple of coupons set up, which work great with logged in users. However, if a guest user tries to use a coupon, they get the error screen (with the dropped ice cream with the message something went wrong). It’s not specific to any coupon as you can type anything into the code box and you get the same error.

I’m not seeing anything specific in the error console, and nothing is getting put in the phperrorlog. I’ve had to disable the plugin. Have you seen this before? Any ideas? I presume the ice cream error message is from this plugin?