How to cleanup when lxc-stop fails?

I have a container in weird state:

lxc-info -n sup_sks
Name: sup_sks
State: RUNNING
PID: 263831
lxc-info: utils.c: switch_to_ns: 1090 No such file or directory - failed to open /proc/263831/ns/net
lxc-info: lxccontainer.c: do_lxcapi_get_ips: 1786 No such file or directory - failed to enter namespace
CPU use: 65.37 seconds

The container is mainly dead, only the init process still lives:

ps -ef |grep 263831
root 9183 263831 0 Sep29 ? 00:00:00 [db_archive]
root 212603 107598 0 07:37 pts/38 00:00:00 grep --color=auto 263831
root 263831 263820 0 Sep29 ? 00:00:01 [init]
103 265432 263831 0 Sep29 ? 00:00:00 [sks]

Only the user and pid namespaces still exist:

ls -al /proc/263831/ns/
ls: cannot read symbolic link /proc/263831/ns/net: No such file or directory
ls: cannot read symbolic link /proc/263831/ns/uts: No such file or directory
ls: cannot read symbolic link /proc/263831/ns/ipc: No such file or directory
ls: cannot read symbolic link /proc/263831/ns/mnt: No such file or directory
total 0
dr-x–x--x. 2 root root 0 30. Sep 07:10 .
dr-xr-xr-x. 9 root root 0 29. Sep 01:16 …
lrwxrwxrwx. 1 root root 0 30. Sep 07:11 ipc
lrwxrwxrwx. 1 root root 0 30. Sep 07:11 mnt
lrwxrwxrwx. 1 root root 0 30. Sep 07:10 net
lrwxrwxrwx. 1 root root 0 30. Sep 07:11 pid -> pid:[4026534655]
lrwxrwxrwx. 1 root root 0 30. Sep 07:11 user -> user:[4026531837]
lrwxrwxrwx. 1 root root 0 30. Sep 07:11 uts

lxc-stop and lxc-stop -k both hang forever.

How can I get rid of this container? I have found two other posts mention a similar problem, but non of it explains how to cleanup after such a situation. I would like to be able to restart the container without rebooting the host (at least I would like to postpone the reboot till the next weekend…)