# apt-get update
E: The method driver /usr/lib/apt/methods/https could not be found.
N: Is the package apt-transport-https installed?

# cat /etc/apt/sources.list
deb http://ftp.de.debian.org/debian/ jessie main contrib non-free
deb http://ftp.de.debian.org/debian/ jessie-updates main contrib non-free
deb http://security.debian.org/ jessie/updates main contrib non-free

# cat /etc/apt/sources.list.d/*
deb http://download.owncloud.org/download/repositories/stable/Debian_8.0/ /

# apt-get install apt-transport-https
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following extra packages will be installed:
  libcurl3-gnutls
The following NEW packages will be installed:
  apt-transport-https libcurl3-gnutls
0 upgraded, 2 newly installed, 0 to remove and 2 not upgraded.
Need to get 252 kB/390 kB of archives.
After this operation, 766 kB of additional disk space will be used.
Do you want to continue? [Y/n] Y
WARNING: The following packages cannot be authenticated!
  libcurl3-gnutls
Install these packages without verification? [y/N] y
Err http://security.debian.org/ jessie/updates/main libcurl3-gnutls amd64 7.38.0-4+deb8u9
  404  Not Found [IP: 217.196.149.233 80]
E: Failed to fetch http://security.debian.org/pool/updates/main/c/curl/libcurl3-gnutls_7.38.0-4+deb8u9_amd64.deb  404  Not Found [IP: 217.196.149.233 80]

E: Unable to fetch some archives, maybe run apt-get update or try with --fix-missing?

# cat /etc/apt/sources.list.d/*
deb http://download.owncloud.org/download/repositories/stable/Debian_8.0/ /