The command save writes all variables of the current workspace to disk, if the variables are not specified. When the ASCII format is used, only a subset of the possible types is possible. E.g. how should a nested struct, which contains cell arrays, whose elements are UINT16 arrays, be written unequivocally in ASCII format?!
Either use the binary format -MAT, or specify the variables, which are written and which have the apropriate type. Read doc save for more details.