The plugin is failing to load when you start calibre. I think this is the relevant bit of the debug log
Code:
File "calibre_plugins.tts_to_mp3_plugin.config", line 32, in <module>
File "calibre_plugins.tts_to_mp3_plugin.utils", line 200, in get_voicedesc_from_id
OSError: C:\r\src\src\calibre\utils\windows\winsapi.cpp:126 :Failed to get current voice:[-2147200966] IDispatch error #20026
I haven't seen this error before but it looks as if maybe your copy of Windows doesn't have a default Voice set.
What do you see under Voices (see screenshot) when you go to
Windows > Settings > Time&Language > Speech
and if you press the
Preview voice button on that screen do you hear that selected Voice correctly?