From Online Manual
Why is my New Thread Indicator Missing
When using a language different than English, the new thread indicator might be missing. This is because you installed a custom theme after installing a language. This will cause the indicator to be placed in a different folder, so the image must be moved to your language folder.
To solve this problem, in your forum's directory, navigate to Themes > Your current theme > English and copy the two files (index.php and new.gif). Then, come back to your theme's folder and create a folder (if it is not created already) and name it with the language your forum is using. For example, if you are using Spanish_es UTF-8, then create a folder and name it spanish_es-utf8 and paste the two files into the folder.