Hi Martin.
I turned on logging and will share logs with as soon as interactive window will appear.
Btw maybe it some related to several sessions (tabs) to same host and using same username?
I'm uploading big data by folders with many files in it.

@d3nz: Please attach a full session log file showing the problem (using the latest version of WinSCP).

Hello!
I'll continue this topic if you are not afraid.
I have the same issue with another one (6.1.1 – SFTP – Abnormal program termination).
Each issue appearing randomly from time to time and both (I have to move big data and don't watch process all the time, but when I check progress I see one of this issue or both).
It appearing both on 6.1.1 and 6.2 beta.
On the source side I have Windows Server 2019 std eng and on destination side – rhel x86_64 8.7 w/o any LB & NAT (connection from source to destination is through L2 network).
Screenshots of errors attached.
If it isn't possible to fix this issues is it possible to add option "Always Add" or "Always Update" for workaround of this issue?

@Guast: Does the host key prompt occur on re-key? Is your SSH server load balanced somehow? Can you post a session log file?

I'm getting the "Potential Security Breach" fairly often and repeatedly during the same transfer process when:

1. tens of thousands of small files are being transferred
   
2. hundreds of gigabytes of files are being transferred
   

There are no attacks, compromised systems or administrators playing around with host keys. Since PuTTY won't suppress security dialogs (see https://www.chiark.greenend.org.uk/~sgtatham/putty/faq.html#faq-hostkeys), unattended transfers are stalled until user action in the event of this warning popping up. Can WinSCP add functionality to accept the "risk" (as in "remember my choice") and continue operation?

Ahh, ok, good to know. I think we're all good then, thank you for the reply.

I assume that the RSA key that you refer to is the one you authenticate with. That's different key than the host key.
See Understanding SSH Key Pairs.

If you have multiple SSH servers behind a load balancer, use Add button to cache keys of individual servers one-by-one.

I realize this is not a new subject, but the previous post(s) I've seen on the subject didn't answer my specific problem. I'm hoping that this isn't a MitM attack, but it could be.

Error after the "Warning" is "The server's host key does not match the one WinSCP has in cache..." and I'm given the option to Update, Add, Skip, or Cancel. I did "Skip" two times and was able to get on to the server and everything looked fine, but I'm concerned that I'm passing sensitive information when I use that option.

For some more detail, my server is hosted on AWS with load balancers and an RSA key-pair. However, the error message states "The new ED25519 key details are:...". I'm no security expert, but I believe ED25519 is a completely different protocol from RSA, which makes me think there is something nefarious going on.

Any help would be appreciated.