Hi @immergruen
That error message isn’t specific to WooCommerce. The most common issue with PayPal is with the IPN improperly marking an order as canceled but I don’t that’s the problem here, but can you confirm the IPN is properly setup?
https://docs.woocommerce.com/document/paypal-standard/#section-6
My assumption though is that the issue is likely caused by a theme or plugin conflict. As a first step, could you try switching back to a default theme like Twenty Seventeen and disable all plugins except for WooCommerce to see if this resolves the issue? If so, then re-enable each one at a time until you find the one that’s causing the conflict.