Export as mp3 blocked despite adding to exceptions

I am using Audacity 2.3.0 on Windows 10.

I have exported dozens of files in mp3 but when I upgraded to 2.3.0 I can no longer do so. I can save as an .aup project, but it won’t let me export as mp3.
I have shut down and rebooted the computer.
I have tried turning off both the Private and Public firewalls in Windows Defender…no joy.
I have deleted and re-added the Audacity .exc file to Defenders exception list.
I have downloaded LAME and added its .exc file to the exception list.
When I go to the Edit tab and look in Libraries, under MP3 Library I see: MP3 Library Version: LAME 3.99.5.
The file I’m working on presently is 37 mins long. I saved it as 000temp.aup …no problem. Then when I try to export as 000temp.mp3 it says File Not Found. WTH does that mean?

Second example: I opened an existing mp3 file titled 00-Intro. Then when I go to re-save with the same name (still mp3…I’m simply replace the existing file) I first get the warning asking if I want to replace it, I click yes, then get the Edit Metatags box and click OK without changing anything, I then I get the message “Unable to Export.”

Help please.

I assume you an export as WAV?

I see: MP3 Library Version: LAME 3.99.5.

That looks like the wrong version for Audacity…

Download & run [u]Lame_v3.99.3_for_Windows.exe[/u] to install the special Audacity version.