Thanks for sharing your concern and the troubleshooting steps you've made in QuickBooks Community, @abdomunla. 

Let me share additional steps to help rectify the issue and customize your sales forms.

There are times when too much cache build-up in a browser can cause unexpected behavior with the product. This can be the reason why you're getting a blank page when editing the template.

Let's try accessing your QBO account in a private or incognito window. This will help us check if the issue is cache-related and fix it. Simply press the following shortcut keys to access this mode:
 

• Ctrl + Shift + N for Google Chrome
• Ctrl + Shift + P for Firefox & Microsoft Edge
• Command + Shift + N for Safari

Once you're in, customize again your invoice template. If you're able to connect without any issues, go back to your regular browser and then clear its cache and cookies to start over fresh. 

Just in case, I'll add this article for future reference: Import custom form styles for invoices or estimates.

I want to make sure everything is taken care of for you, so please let me know if you have any other issues or concerns. I'm always here to help. Have an amazing rest of the day!