UE5.1 Packaging Error: System.ArgumentOutOfRangeException: startIndex cannot be larger than length of string. (Parameter 'startIndex')

[anon86204179]

I faced the same issue. My workaround is to disable “Use Pak File” and “Use Io Store” in Project Settings. (You may only need to disable one of these.) After that my project built just fine.

But still, i don’t know what really causing the issue.