I get the following error when trying to upload via SFTP:
"1003: could not negotiate client MAC algorithm using “hmac-sha1, hmac-md5, hmac-sha1-96, hmac-md5-96”

And when trying to see online website files locally in HTML Editor:
"1001: Other end disconnected during key negotiation"

Using another FTP client works as expected, so this must be an issue with HTML Editor. I have check the credentials and, and this connection has been working for several months. I didn't change anything, but now I cannot upload.

Any thoughts?

Try the following. Add the HTML editor to all possible exclusion lists in both the antivirus and the firewall, and most importantly, be sure to restart the system afterward. If that doesn't work, check the Task Manager to see which processes are associated with the HTML editor and add them to the exclusion list. Everything should work after that.

Unfortunately that did not solve the problem.

Joel, there hasn't been any changes in the HTML Editor nor the CC DFTP for quite some time. Could it be in your OS, or at your Web host? Or in your security software?

Well then, the last thing left is to contact technical support.

Yeah I did that weeks ago. While they tried to be helpful and I have no complaints, they said that the HTML editor has not changed in a long time and so there's nothing they can do (which is fair).

One day FTP upload worked, the next day it didn't. I can upload just fine with the same FTP credentials with FileZilla. I suspect this started happening after a Windows update.

The frustrating part is that the ability to upload via FTP from within an HTML editor was THEE reason I was willing to pay for the software. And now that reason is gone.

Yes, I understand you. It is for this reason that the search led me to this site. I was looking for an HTML editor with the ability to edit the code on the hosting. Before this, I did this with the free WinSCP FTP client, which allows this.
Maybe the developer should still update his HTML Editor, because there is still a huge difference between it and SD. Even though I bought the software package just a week ago, I’m ready to buy HTML Editr again if the developer updates it.
If your hoster allows you to select the FTP protocol on the server, then you can select standard FTP. I don't think there will be any problems with him. You can enable this protocol for a while, and then re-enable SFTP for security purposes.

Have you tried updating it?

Update what? From what I understand HTML editor hasn't been updated in a long time and I'm running the latest version.

euroaliment.ugal
is a guest so that means he doesn't own any CC software, therefore he is just trying to help as a guest with a generalised suggestion rather than an informed one and can't see what the latest or earlier versions of HTML editor are.