Fix for 'Invalid base64 string for MIME content' in Ventura

MIME is a standard for including non-ASCII text and attachments in e-mail messages. Base64 is a way of encoding arbitrary binary data using ASCII characters to allow transmitting it over channels that only support text.

Wikipedia – MIME

Wikipedia – Base64

It sounds as if you may be trying to read an email message that is corrupted or improperly-formatted. If the copy of the message that you received is corrupted, I do not believe that there would be any magic "fix". You would need to get another, good, copy from the sender.

Goggle search suggests that this issue may come up for Apple users attempting to connect to Microsoft Exchange accounts.

Invalid base64 string for MIME content - Apple Community

Solution for Invalid base64 string for MI… - Apple Community

I think you're going to have to provide just a bit more information.

Where is this message appearing? In what app? in response to what actions?

One time? Constantly? What options do you have in response to the message? What happens when you acknowledge it?

As it stands, you're asking us to shoot at 2mm target buried somewhere on a football field, in the middle of the night, blindfolded, with our arms tied behind our backs.

You need to paint a bigger picture here.

Writing an effective Apple Support Communities Question - Apple Community