I appreciate the steps you've taken, usermorita. Let's work together to make sure you can change the invoice email subject line of your account.

I've tried this on my account and I'm able to change it without getting an error. We can figure out if this has something to do with your browser's cache. Sometimes, excessive cache files can cause unexpected responses from QBO.

Please try changing your template using a private browser to see if you can successfully change it. You can use these keyboard shortcuts:

• Google Chrome: Ctrl + Shift + N
• Mozilla Firefox: Ctrl + Shift + P
• Safari: Command + Option + P

If it works, let's go back to your default browser and clear its cache. It removes browsing history causing glitches on your browser. Otherwise, use other supported browsers as an alternative.

To learn more information about customization, you can check out customize invoices, estimates, and sales receipts in QuickBooks Online.

If you're still getting the same results, I'd recommend contacting our Customer Care Support Team so they can further check on your account and troubleshoot it for you.

Keep on posting if I can be of any additional assistance concerning customizing your templates. I'll be happy to help.