Error: Authentication failed on fileserver (Error code: 29115) observed on scheduled and manual backups.

Products

• VMware NSX

Issue/Introduction

• A Windows server is configured in NSX as the SFTP server.
• Both scheduled and manual backups are failing with error:
  Error: Authentication failed on fileserver (Error code: 29115)
• In NSX Manager's /var/log/syslog log file, an error similar to the error below can be observed:
  2026-01-19T13:03:29.861Z <nsx-manager> NSX 2181 - [nsx@6876 comp="nsx-manager" subcomp="node-mgmt" username="root" level="ERROR" errorCode="NOD110"] Node backup file copy operation failed due to 400 Bad Request#015#012Content-Type: application/json#015#012Content-Length: 158#015#012#015#012{"error_code": 36222, "error_message": "The remote server returned error: status: 255   out: '[Errno 5] Input/output error'.", "module_name": "node-services"}
• SSHD logging on the SFTP server indicates "Connection aborted":
  9904 2026-01-27 10:45:02.848 Connection from <NSX-manager> port 48940 on <SFTP-server> port 22
9904 2026-01-27 10:45:02.848 error: kex_exchange_identification: read: Connection aborted
9904 2026-01-27 10:45:02.848 banner exchange: Connection from <NSX-manager> port 48940: Connection aborted
• Packet capture done while the backups are triggered (and fail) indicate a failed SSH key exchange due to server unresponsiveness (server doesn't reply with it's own SSH key). 
  There will be multiple key exchanges while the backup routine is run.
  08:44:47.845512 IP <nsx-manager>.48940 > <sftp-server>.22: Flags [S], seq 2096795560, win 64240, options [mss 1460,nop,nop,sackOK,nop,wscale 7], length 0
08:44:47.845935 IP <sftp-server>.22 > <nsx-manager>.48940: Flags [S.], seq 3862630903, ack 2096795561, win 65535, options [mss 1460,nop,wscale 8,nop,nop,sackOK], length 0
08:44:47.846058 IP <nsx-manager>.48940 > <sftp-server>.22: Flags [.], ack 1, win 502, length 0
08:44:47.846924 IP <nsx-manager>.48940 > <sftp-server>.22: Flags [P.], seq 1:43, ack 1, win 502, length 42: SSH: SSH-2.0-OpenSSH_8.9p1 Ubuntu-3ubuntu0.13
08:44:47.857066 IP <sftp-server>.22 > <nsx-manager>.48940: Flags [.], ack 43, win 8195, length 0
   >>> here, we expect another packet with SSH key exchange, sent by the sftp-server to nsx-manager <<<
08:44:50.849621 IP <nsx-manager>.48940 > <sftp-server>.22: Flags [F.], seq 43, ack 1, win 502, length 0
08:44:50.849896 IP <sftp-server>.22 > <nsx-manager>.48940: Flags [.], ack 44, win 8195, length 0

Environment

VMware NSX

Cause

SSHD on the SFTP server is not responding to SSH key exchange, which leads to termination of connection, and NSX backup fails.

Resolution

This issue needs to be addressed on the SFTP server side. 

Recommendations are to ensure that:

• You use SFTP server build on supported OS with OpenSSH version, listed in Configure Backups. 
• There isn't more than one instance of SSHD installed.
  To validate - in PowerShell, run the command below. If it returns more than one path, you have a conflict, and one installation of SSHD needs to be removed. 
  where.exe sshd
• MaxStartups value in sshd_config file on the SFTP server may be throttling the sessions. To validate, you can comment out the line and restart SSHD service, and see if the issue is still reproducible.
• DNS server may be slow to respond, or unreachable. To eliminate this as a possible reason, you can disable DNS resolution by uncommenting the line below in sshd_config file on the SFTP server, and restart the SSHD service:
  # UseDNS no
• If further troubleshooting is required, please engage the Microsoft support team to investigate the issue from the perspective of SFTP server. 

Additional Information

If you see this error, also review KB articles:

• "Authentication failed", "bad directory" or "server disk full" error when preforming NSX backup to Windows SFTP server
• Authentication failed on the fileserver for SFTP backups on NSX Manager UI: Error code: 29115