Hi @mike156,
Thanks for your question.
As this challenge is preventing sales at checkout, the Afterpay team would advise disabling the plugin immediately, if you have not already done so. Next, please consider taking a backup of the site (database and files), then rolling back to the previous version of the plugin (currently v3.5.5).
The team would like to assist in getting the latest version running correctly. To facilitate this, please submit a support request including a link to this post using the following web form: https://afterpay.formstack.com/forms/global_merchant_technical_support
A member of the team will then reach out to you within 1 business day.
Thank you.