I appreciate the troubleshooting steps you've done, yedallc.
We can open an incognito window and create the invoice from there. Also, all browsers saves junk files called cache which can cause issues in QuickBooks Online. We can clear the them so we can resolve the invoicing error.
Aside from that, make sure that your browser is updated since old versions can also result to some errors. You can use a different browser like Google Chrome, Mozilla Firefox or Microsoft Edge.
I'd like to share these articles about customer payments:
Please comment below if you need more assistance. I'll be happy to help.
