Created attachment 1838 [details]
strace -f -F -o sftp.log -s128 -p <sshd pid>

Sftp doesnt work with scponly with debuglevel 
(echo 2 > /etc/scponly/debuglevel)
if debuglevel is set to 0 than sftp works.

I think that problem begin on line 1645:
1645 31404 pipe([11, 12])                    = 0
1646 31404 clone(child_stack=0, flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD, child_tidptr=0xb7c5c728) = 31405
1671 31405 close(11 <unfinished ...>
1690 31404 close(11 <unfinished ...>
1898 31405 writev(2, [{"scponly[31405]: chrooted binary in place, will chroot()", 55}, {"\n", 1}], 2) = -1 EPIPE (Broken pipe)
1899 31405 --- SIGPIPE (Broken pipe) @ 0 (0) ---

see full strace log in attachment

Behavior of sftp is sometimes random:

I have reproduce with scponly in chroot, if I have change shell from 
scponlyc to scponly, sftp doesnt work too, but after restart of sshd sftp works.

sometimes works
sftp testik@192.168.0.108
testik@192.168.0.108's password: 
Connected to 192.168.0.108.
sftp> 

sometimes whithout any changes doesnt work
sftp testik@192.168.0.108
testik@192.168.0.108's password: 
Connection closed

I am not sure if this bug in openssh or in scponly or somewhere but closing of pipes are before calling /usr/local/sbin/scponlyc.
But reading of debuglevel is after closing pipes.

I have reproduce this with openssh5.4p1 and openssh5.3p1 too, with scponly-4.8-r4 on gentoo linux.