Did you tried to download it again and replace the one in your Program Files (x86)\Steam\steamapps\common\Arma 3 ?
Or you can try it this way:
Unpack your [mapname].pbo that you are using for you server. I use PBO manager to do it. You might be using something else.
In your CUP folder, look for description.ext
Open the description.ext in notepad and search if there is a line that says:
"\cup\terrains\cup_terrains_core\cup_terrains_logo_mod.paa"
I had this problem with Exile servers, and changed the .paa but I wouldn't touch 506th Files tbh. Just look up if it's included otherwise it's a corrupted download (possible)