Resuming apt-mirror fails due to corrupt index files
Solution 1:
Solution for "...gzip: stdin: not in gzip format...":
I use Ubuntu 16.04 LTS 64bit version. You need to delete "skel" folder ONLY than run apt-mirror again!
$ sudo rm -rf /var/spool/apt-mirror/skel/
$ apt-mirror
I don't know why but it works for me.