You must enable large file support to be able to use libexplain. If your project uses GNU Autoconf, it is simply a matter of adding the AC_SYS_LARGEFILE macro to your configure.ac file. If not, you need to add -D_FILE_OFFSET_BITS=64 to your CFLAGS definition (this is not completely portable, consider using GNU Autoconf instead).
Large file support is needed in order to be able to access files larger than 2GB in size. Some fundamental data types, such as off_t, are defined differently, and this has many follow-on effects. A libexplain design decision was to support large files, and this in turn means that clients of libexplain must also enable large file support, so that libexplain and its client programs are using the same data sizes. (Failure to do so leads to hard-to-find memory scribbles and segfaults.)