This non-QIIME error usually occurs if the file became corrupted, e.g., during gzipping your fastq sequences.
Does that mean that you tried dividing and re-importing the sequences and the error no longer occurred? That would make sense if the initial gzip file became corrupted.
You could also try qiime tools validate paired30.qza to check that file before demultiplexing.
Please let us know if you are still having trouble. If so, please post the output of validate.