Why is there ‘unexpected error’ message showing when clients open invoice to pay?

Thanks for reaching out to the Community, @henna.

There are times that the browser is full of frequently accessed page resources, causing some unusual responses. As an initial troubleshooting step, have your clients open the invoice to pay using a private browser (incognito).

Here's how:

• Press Ctrl + Shift + N (Google Chrome).
• Ctrl + Shift + P (Firefox or Internet Explorer).
• Control + Option + P (Safari).

If it works, have them perform a clear cache on their default browser. Every so often the cache becomes overwhelmed with older data which can potentially cause viewing and performance issues. Clearing it will refresh the system, and they'll be able to work with a clean slate. However, if the issue persists, have them use other supported browsers.

Just in case, I'll add this article for future reference: Create an invoice your customer can pay online.

Please let me know if you have any other concerns or issues in the comment section below. I'm more than happy to help. Have a great weekend!