Issue Scheduled backup via FTP - error message even when successful

[Alban Staehli]

TITLE:
Scheduled backup via FTP - error message
PRODUCT, VERSION, OPERATING SYSTEM, ARCHITECTURE:
Plesk 17.5, CentOs 7.3
PROBLEM DESCRIPTION:
All of my Scheduled backup using FTP report an error each time they're being executed:

Unable to access to the storage: Transport error: unable to list directory: Curl error: (67) Login denied: Last FTP request: PASS ***** Last FTP response: 530 bad login

Nevertheless, the backup files have been correctly generated and are available on the FTP server - despite this error notification.
I believe there shouldn't be any error notification then.
STEPS TO REPRODUCE:
Schedule a backup using FTP repository (FTP repository using PASV and NOT SFTP, with email notification in case of error). Let it run by itself when scheduled, and check your email.
ACTUAL RESULT:
Error notification being received for each and every scheduled backup made by FTP, even do the process appears to be succesful.
EXPECTED RESULT:
Shoud not send error notificataion.
ANY ADDITIONAL INFORMATION:
none
YOUR EXPECTATIONS FROM PLESK SERVICE TEAM:
Confirm bug and fix it

 

[Bitpalast]

We've been using automated backups intensively for years and have never seen this error. We are not seeing the error on different 17.5 installations either.

The backup procedures have changed between the past versions of Plesk. They became more flexible. It is thinkable that Plesk tries to open several FTP connections at a time.

"530 login denied" is not only caused by wrong access credentials, but it can also be caused by too man concurrent login attempts. For example when your FTP storage only allows 1 concurrent FTP connection and you try to open a second one from the same source, the second one will be denied access. I suggest to manually test this by opening two or three SSH sessions to your source server and from there try to open three concurrent FTP connections to your FTP storage server to validate, whether this could be the cause.

 

[Alban Staehli]

Thx for this very useful answer Peter!
I can confirm the FTP server I'm using has a limit of 6 concurrent connections - therefore I've set Plesk to run a maximum of 6 backup processes at a time.
Moreover, as mentionned, the issue here is that the files are being generated and copied to the FTP server - but an error notification is still being sent.
So maybe the Plesk scheduled FTP backup procedure retries the transfer after an error? Could you confirm that?