There are no errors in the test cases. Control your program.
On some tests you have a problem. I tried your solution (after completing things that were missing). You don't have thought of particular cases and on these cases your code fails and give you

System.IndexOutOfRangeException : Index was outside the bounds of the array.

Write a function to print the input (a long[,] array) and then I think you will be able to see the flaw in your code.
Consider yourself as happy: usually I don't examine the code... I don't have time for that. I hope you will succeed.
PS: here a markdown guide, in particular for showing your code:
https://github.com/adam-p/markdown-here/wiki/Markdown-Cheatsheet#code

I don't need your code, I don't have enough time. Rather give me (between 2 lines of 3 backticks, otherwise it is unreadable; if you don't know that, google "markdown") what you exactly get when the tests fail.

In test1 of "Run" (Your test cases) there is only one "subtest". In "Submit" there are 8 tests. You can succeed in subtest1 of test1 but fail in one of the 7 other tests. Print the input to see what is going on in your code which caould have a bug. Be sure that there are no errors in the tests.

I just tried a few solutions at random amongst the passing solutions and everything worked fine. I can't see any error in test 1. The tests are the same in all languages and lots of Codewarriors passed the kata in C#.

With the previous version of C# at Codewars it passed but the new version is very strict... I suppressed the unused line, should work now.