Issues with a weird looking font can be caused by a problem with a specific font and you need to identify this font and fix this. You may find that it can be resolved by finding and removing duplicate fonts installed in the local ~/Library/Fonts folder, that are conflicting with standard system fonts.

You can check in Font Book for font issues like corrupted and duplicate fonts.

Note that you shouldn't disable "Allow pages to choose their own fonts, instead of your selections above" as this will cause issues with iconic fonts used by webpages to display small icons (you may see text labels instead of icons).

thanks cor-el for your answer.

I couldn't see any duplicate or corrupted fonts but after removing my (only) private font the mail got displayed correctly.

But now I get an error in a different application (jconsole) because the removed font is missing :D. The font in question is "Times" which I installed manually since MacOS only has "Times New Roman".

any ideas on that?

Thanks for your help