Some plugins are missing and then there is an 524 error.
# ipsec --version
Linux strongSwan U5.5.0/K4.7.9-64
Institute for Internet Technologies and Applications
University of Applied Sciences Rapperswil, Switzerland
See 'ipsec --copyright' for copyright information.
# ipsec start
Starting strongSwan 5.5.0 IPsec [starter]...
# ipsec up not-shown
no XAuth method found
establishing connection 'not-shown' failed
No surprise, the xauth plugin is missing.
# aptitude update
# aptitude install libcharon-extra-plugins
The following NEW packages will be installed:
0 packages upgraded, 2 newly installed, 0 to remove and 2583 not upgraded.
Need to get 485 kB of archives. After unpacking 1,508 kB will be used.
The following packages have unmet dependencies:
libcharon-extra-plugins : Depends: libstrongswan (= 5.2.1-6+deb8u2) but
5.5.0-2 is installed and it is kept back