According to the KB and my own experience, there is more than 5 years people are complaining about the "Index was out of range. Must be non-negative and less than the size of the collection. Parameter name: index" error, and after multiple new versions and SR's, the errors still happens.
The error as such is already an issue, but what adds insult to injury is that one becomes unable to insert segments from memory in the subsequent segments until Studio is restarted. It is the 5th time I restart Studio this afternoon!
I work on a series of DOCX documents, and I own Studio 2021 SR1 (SR2 causes issues with Groupshare at this point, so I wait for that issue to be fixed).
I think resolving that long time issue should be a priority for SDL team. At least preventing the error to interfere with the ability to insert segments from memory after the issue happens.
Regards