I'll share some details of what may have caused this error and the basic troubleshooting steps to fix this.
Sometimes, this error can be caused by an outdated and corrupted cache stored in your browser. To resolve this, let's open your account in an incognito window or private browser. This mode will not save your browsing history, cookies and site data, or other information.
Here are the keyboard shortcuts:
Google Chrome: Ctrl + Shift + N
Mozilla Firefox: Ctrl + Shift + P
Safari: Command + Option + P
After signing in to your account, open an invoice and try sending it. If you can successfully send the invoice using the private/incognito browser, then you'll want to clear your regular browser's cache. Once done, please make sure to restart your browser.