I just chatted through my QB Online account and gave the CS rep the investigation number. Not sure what she did, or if it just fixed while I was online with her, but I am now able to save my invoices.
Confirming the same as well; looks like the technicians resolved the issue. Thanks everyone for bringing this to the forefront for QB/Intuit, and @Mark_AG for the status update earlier!
Thank you for visiting the QuickBooks Community. I'm glad that the issue you're experiencing has been resolved. Also, It's great to know that my colleague's update was able to help you.
Here at QuickBooks, we're committed to providing you with the best possible customer service. Feel free to get in touch if you need help with anything else in QBO. We're always here to assist you. Have a great day!