Vortex is right, this is not the place to ask for a DosBox question.
However, I'll throw you a bone anyway. The executing file probably want to call '_SDL_strlcat' but cannot find it. Possibly because the version of sdl.dll is different. I am guessing DosBox is using it's own compiled version of SDL that exports a decorated 'SDL_strlcat' (hence the: '_SDL_strlcat'. Notice the underscore!) This is what it is looking for (and I am guessing here) Which is why you can't find it (and I am guessing again) because you took a version of SDL.DLL from the SDL site?