If you ever encounter the issue of missing style icons/buttons (such as new topic, post reply, locked, etc.), then this article is for you.
It is not really hard to fix this issue - basically, you need to add a new imageset for your style, one that is relevant to your boards language.
So for example, if your board default language is Dutch (NL), you need to create a Dutch imageset.
If in this case the imageset is different from the default prosilver/subsilver2 styles, your solution would be to duplicate the existing imageset.
So for example: if the existing imageset contains a folder called "en" and you are using the language "en_us", then you need to duplicate the "en" folder (including all of it's contents) and rename it to "en_us". You should now have two folders - "en" and "en_us". This allows you to use either depending on the boards language.