WinSCP sometimes throws error - Server refused public-key signature despite accepting key

Question

I have a process scheduled to send files to an SFTP server (Tumbleweed) using WinSCP.

It works 99.5% of the time.

0.5% of the time, the login fails and WinSCP reports:

Server refused public-key signature despite accepting key

WinSCP Log Excerpt:

! 2013-04-07 12:05:18.913 Using username "XXXXXX".
. 2013-04-07 12:05:18.928 Offered public key
. 2013-04-07 12:05:19.460 Offer of public key accepted
! 2013-04-07 12:05:19.460 Authenticating with public key "XXXXXXXXX"
. 2013-04-07 12:05:19.491 Sent public key signature
! 2013-04-07 12:05:19.491 Server refused public-key signature despite accepting key!
. 2013-04-07 12:05:19.491 Server refused public-key signature despite accepting key!
. 2013-04-07 12:05:19.491 Attempting keyboard-interactive authentication

The server logs no error:

2013-04-07 12:05:19,816 DEBUG [SSHD-TRANSFER-1] com.tumbleweed.st.server.sshd.SSHSession - Adding session xxxxxxx. Session count: 0
2013-04-07 12:05:19,816 DEBUG [SSHD-TRANSFER-1] com.tumbleweed.st.server.events.AgentSession - Created unique session ID: xxxxxx 
2013-04-07 12:05:19,816 DEBUG [SSHD-TRANSFER-1] com.tumbleweed.st.server.sshd.PublicKeyStoreImpl - invoke cert agent
2013-04-07 12:05:20,350 DEBUG [SSHD-TRANSFER-1] com.tumbleweed.st.server.sshd.PublicKeyStoreImpl - cert agent success [username=xxxxxx]
2013-04-07 12:05:20,386 DEBUG [SSHD-TRANSFER-1] com.tumbleweed.st.server.sshd.PublicKeyStoreImpl - User "xxxxx" logged in with cached key.

How can I diagnose / resolve this issue?

Answer 1

This is known bug in proftpd:

http://bugs.proftpd.org/show_bug.cgi?id=3992

The fixes has not been released yet. It's available only as a patch to the latest release.