The header of each XTC frame contains this number to designate it as an XTC frame. For whatever reason, your file has completely different data there. If this happens half-way through the file, you can still rescue the earlier parts by reading and then writing to another file.
Unfortunately this means your file (i.e., disk storage) is corrupted in a major way. This can be due to a number of reasons, all of which are not part of GROMACS (Bad disk, bad controller, corrupted file system, multiple processes writing to the same file without proper locking, etc.)