pure-ftpwho works partially:

# ps ax |grep pure-ftpd | grep -v PRIV| sed 's/ *//g'
741?S0:00pure-ftpd(IDLE)
758?S0:00pure-ftpd(IDLE)
773?S0:00pure-ftpd(IDLE)
788?S0:00pure-ftpd(DOWNLOAD)
821?R0:02pure-ftpd(DOWNLOAD)
828?S0:00pure-ftpd(IDLE)
844?S0:00pure-ftpd(DOWNLOAD)
3444?S3:32pure-ftpd(DOWNLOAD)
4141?S0:47pure-ftpd(DOWNLOAD)
13126?S0:47pure-ftpd(IDLE)
21824?S0:48pure-ftpd(DOWNLOAD)
24388?Ss0:00pure-ftpd(SERVER)
30924?S0:14pure-ftpd(DOWNLOAD)
30925?S0:12pure-ftpd(DOWNLOAD)
31597?S0:05pure-ftpd(DOWNLOAD)
31603?S0:05pure-ftpd(DOWNLOAD)
32310?S0:10pure-ftpd(DOWNLOAD)

# pure-ftpwho -s
844|somesert|56| DL |somefile.iso|LRouen-xxxxx.abo.wanadoo.fr|xxx.yyy.org|21|25600|711902|4|465

Only one user is present. If I retype pure-ftpwho -s some second afer, another line is showed.

But process 844 is still here.
# ls -l /proc/844/fd
total 0
lr-x------ 1 root root 64 May  2 18:12 0 -> /dev/null
l-wx------ 1 root root 64 May  2 18:12 1 -> /dev/null
lrwx------ 1 root root 64 May  2 18:12 10 -> socket:[1401326]
lr-x------ 1 root root 64 May  2 18:12 11 -> somefile.iso
lrwx------ 1 root root 64 May  2 18:12 12 -> socket:[1401370]
l-wx------ 1 root root 64 May  2 18:12 2 -> /dev/null
lrwx------ 1 root root 64 May  2 18:12 3 -> socket:[1401323]
lrwx------ 1 root root 64 May  2 18:12 4 -> socket:[1091451]
lrwx------ 1 root root 64 May  2 18:12 5 -> socket:[1091453]
lrwx------ 1 root root 64 May  2 18:12 6 -> socket:[1401322]
lrwx------ 1 root root 64 May  2 18:12 7 -> /var/run/pure-ftpd/client-00000844 (deleted)
l-wx------ 1 root root 64 May  2 18:12 8 -> /var/log/ftp/transfer.log
lrwx------ 1 root root 64 May  2 18:12 9 -> socket:[1401367]

Problem is that the state file has been deleted. Why?

I always have at most 1 file in /var/run/pure-ftpd.