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)
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.