Upgraded my Square module to the WebPay module last week, was working great, taking charges with no issue.

Today I got a debug file with this at the end:
PHP Notice: missing token result: check if card button is disabled on dev console using: document.getElementById('card-button'); else look for javascript error in console. in /usr/www/users/xx/includes/modules/payment/square_webPay.php on line 261.

Customer abandoned their cart at checkout_payment.

I saw some other messages about this, but must have missed the solution. How can I fix this?
Using 1.56c, php 7.3, standard checkout flow, daily cron task for handler update.

many thanks,
Ed