FTP backup using ftpes fails

[flexserve]

3CX v18.8 Pro - Debian, cloud hosted.
FileZilla Server 1.7.3 on WS2016, commercial DV SSL cert, behind gateway router, open port no translation

Issue: FTP backup works correctly, FTPES errors trying to read folder, unable to backup or restore.
Verified FTPES works inside/outside FTP server network with both FileZilla client and WinSCP.

After changing protocol from ftp:// to ftpes:// upon saving dialogue 3CX reports "Backup location configuration error", no previous backups dislayed.
The session log on FZ reports the following error: "425 Unable to build data connection: TLS session of data connection not resumed."

Does anyone else have FTPES working? Grateful for any details that you might share. I have seen many posts with same issue but no resolution listed.

TIA

 

[TheodorosG_3CX]

There are some posts that might be helpful to resolve your issue:
• https://wiki.filezilla-project.org/Setting_up_your_Router_to_Fix_the_"425_code"
• https://forum.filezilla-project.org/search.php?keywords=TLS+resumption