OK, I actually saw only the last instance of the error. I didn't notice that some earlier instances also had the "Received SSH2_MSG_CHANNEL_DATA for nonexistent channel 0".

Well, the two problems occur at the same time. One on the SFTP level and one of the SSH level. It's hard to decide which error should take precedence. I do not know why you server sends SSH2_MSG_CHANNEL_DATA on nonexistent channel 0.

Hi,

You are right, both logs show the same error message. However, there is an inconsistency between what is logged and what it is displayed when uploading files. And it seems to be related to the size of the file.
I will attach printscreens for demonstration.

Both logs show

Your quota has been exceeded

Hello,

I am posting verbose logs from both WinSCP and FileZilla.

Please post verbose logs from both WinSCP and FileZilla showing different messages for the same file.

Yes, with FileZilla the error message is consistent every time the quota is exceeded.
Even when I am using small or large files the same error message is shown.

Do you have different experience with other SFTP clients?

Hi,

When quota is full over the remote system where the files are stored the error message should be always the same, instead it changes to

Received SSH@_MSG_CHANNEL_DATA for nonexistent channel 0

This is confusing for users because it doesn't say explicitly what is the issue.
When uploading with smaller files then the error message is handled correctly and it display:

General failure (server should provide error description).
Error code: 4
Error message from server: ERROR: Your quota has been exceeded. Available: 0k.

After doing this for a couple of times, it will request the password for the user connecting even though the user is using a key based authentication.
WinSCP version is 5.19.2 (Build 11614)

Is there something I am missing here?
How can I get the same error message when the quota is exceeded because the server does provide an error description the way it should.

Thank you,
Traian