I guess your platform is linux as I have this issue here. Some source files reference a file with _Anim with a capital A while the files on disk do not and vice versa. Would be nice if the dev can fix his scripts. A workaround is renaming the files to match the capital (or lowercase) 'A' and pressing reload. There are usually several animation files in a row that are wrong. So, while you're in the www/ directory, with:
basename=Opt_NyxM_Licking
# ..or whatever basename you get the error for
ls -1 img/pictures/${basename}_anim*.png | while read f; do mv "$f" "${f/_anim/_Anim}"; done