Pending your getting a further response from the CoffeeCup team, and if it hasn't been tried already, it might be worth doing a test upload to the site using an FTP program to see whether that triggers the same error message. If it doesn't, that might of course provide a temporary, though less convenient, solution and might indicate that the problem lies with Site Designer 4 build 3113.
Although I use SD for my website, I always export updates to my local hard drive for testing, and then upload from the hard drive to my web host using DirectFTP.
If you want to try an FTP client, I think you need to use CC's DFTP.
I reported your case to CC and asked them to investigate. I believe that you may have got a response by now, saying that the cause might be in your computer, that you perhaps have some old TLS (Transport Layer Security) file that causes the error message. They tested your site by uploading through both SD and WFB without errors.
It might well be that this error goes away if you uninstall your old version of SD, reboot your machine, and then install the latest version.
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