Hello there, GregoryD.
I appreciate you for letting us know your experience with the support. This is something Intuit doesn't want you to come across in your future interactions.
I'm adding some troubleshooting steps to fix the issue with emailing invoices as well as switching to a single-user mode in QuickBooks Desktop.
If you haven't tried running the QuickBooks File Doctor from the QuickBooks Tool Hub, I suggest doing so. This helps fix common errors. QuickBooks needs to be closed to use the tool hub.
For additional troubleshooting options when fixing Unrecoverable Errors, kindly check out the following articles:
If you need other helpful articles in the future, you can always visit our site: Help articles for QuickBooks Desktop.
Let me know if how this goes and if there's anything I can help you with. I'll be around. Have a good one.
