Can you send me an email, so I can send you back a debug version of WinSCP to track the problem? Please include a link back to this topic in your email. Also note in this topic that you have sent the email. Thanks.

I'm having the same issue, also happens in 6.3.3
It's very annoying as the S3 site is the first thing WinSCP automatically selects on startup so the UI immediately froze.

6.1.2 is working. I had to redact some information on image. I concede this morning after frustration last night, if you don't click as to not have the windows intervene and wait many minutes it will begin responding.

Can you please post some screenshots documenting the problem? Or even better record a screencast?
https://winscp.net/eng/docs/reporting#recording
Which old version does work?

I installed the new version. I clicked on my Amazon S3 entry in site manager. But after clicking on it, the site manager behaves weird. It will not let me select a different site. It seems to still have the Amazon S3 site selected while also my trying to click open a different site. It saved that as my last site so that I restart app after crash, I can not click on any other site. I had to install old version to fix.