Does vsftpd retry without CLONE_NEWNET if attempt with CLONE_NEWNET failed?
If yes, reenabling CONFIG_NET_NS with this (untested) patch might help.
Even if no, reenabling CONFIG_NET_NS with this patch with
adequate initial value (e.g. 64) assigned to max_netns_count might help.
Does vsftpd retry without CLONE_NEWNET if attempt with CLONE_NEWNET failed?
If yes, reenabling CONFIG_NET_NS with this (untested) patch might help.
Even if no, reenabling CONFIG_NET_NS with this patch with
adequate initial value (e.g. 64) assigned to max_netns_count might help.