SFTP file transfers fail since update

[Rattle22]

I am trying to back up to my SFTP as I have in the past, but a lot of the transfers fail.
The first 8 (of 8 parallel channels) fail with these two errors:

LIBSSH2_ERROR_TIMEOUT: Der Vorgang hat das Zeitlimit von 30 Sekunden überschritten. [libssh2_sftp_write]
(Roughly translated form German: LIBSSH2_ERROR_TIMEOUT: The operation exceeded the 30s time limit. [libssh2_sftp_write])
LIBSSH2_ERROR_SOCKET_SEND: Unable to send FXP_CLOSE command [libssh2_sftp_close]

Then, the rest fail with this error:

LIBSSH2_ERROR_CHANNEL_FAILURE: Unable to startup channel [libssh2_sftp_init]

The files this happens to aren't different from the ones I transferred in the past - similar names, total path lengths, destinations etc.
Some files have transferred properly, and it seems to happen to all remaining files in my setup.

FileZilla does not have problems transferring any of files to the server.

I have tried increasing the timeout for sftp file transfers to 120 seconds, without success. It was 10s before, so I don't think the 30s in the error message are related to that.
I tried using wireshark, but I don't have the necessary experience to deduce anything useful. I am seeing a lot of packages with invalid structures, but I have no idea if this is related to the issue at all or just normal noise from transfers.

This has only happened since I last updated, and I don't know either what version I last had, nor do I know how to get old donators versions from the website.

[xCSxXenon]

You can't get an old donation edition from the website, but it is still advisable for your to install the last free version to test if it starts working again. If it does, update to the latest and test again. If it breaks again, it is likely a FFS issue

[Zenju]

Also: On which operating system does this happen?

[Rattle22]

So, I tested it with 12.6, and there the file transfers did seem to complete.
However, I identified a upload speed degradation (from multiple mb/s to about 200 kb/s) with my provider that went away by the time I got around to doing something about it. Without the speed degredation, everything works as expected. So in a sense you can consider this resolved.