The latest version supports any Lemmy 0.18.3 instance. You can see which version a Lemmy instance is at the bottom of site. For example, your instance, reddthat.com, is 0.18.3. Most instances should have upgraded to 0.18.3 already - choose one of those. Any instance running 0.18.3 should work, big or small.
The account you "download" with won't be touched at all. The account you "upload to" will receive all the subscriptions and blocks of the downloaded account.
Could be some kind of new bug, but I've also seen this happen when an instance is under heavy load or otherwise glitching out - it's starts sending back invalid items on the API and LASIM chokes on them.
For what it's worth I made an account on reddthat.com and was able to download and upload to it fine.
Maybe try again? Or is this some other instance? (I'm making assumptions based on your account)
I will try this again tomorrow and see. Also, when I hit download, the window below that showed logging in successful (don't remember exact words) after that the whole application window vanished. I tried this twice with same result. Later I found that error file. Maybe something in my os might be blocking it? Later when I have time I try it on my old laptop which is on windows 10 and see what happens.
So I tried again. I deleted the application folder then downloaded it again and executed it. Got the same error message. Then did the same on my old laptop on windows 10. This time the application didn't execute giving me a message that some dll file is missing. Looks like this is not going to work for me for whatever reason.
That's so odd. No idea what would be causing that on Windows 10.
For the windows 11 issue I can only think short downtimes of the instance you are trying to use, or your account has something unique about it that I've never encountered before.