Okay… fixed it.
I added this in class-plantuml-renderer-public.php after line 70.
I would have pushed it to a git repository, but I couldn’t find one.
$new_content = str_replace( '‘','\'',$new_content ); //
$new_content = str_replace( '…','...',$new_content ); //
$new_content = str_replace( 'Ä','?',$new_content ); //
$new_content = str_replace( 'Ö','?',$new_content ); //
$new_content = str_replace( 'Ü','ü',$new_content ); //
$new_content = str_replace( 'ä','?',$new_content ); //
$new_content = str_replace( 'ö','?',$new_content ); //
$new_content = str_replace( 'ü','ü',$new_content ); //
$new_content = str_replace( '<br />',PHP_EOL,$new_content ); //
