I was wondering, whether it makes a difference whether the constructor or the solve method throws the exception on invalid inputs. But that does not seem to be relevant: I tried to do all validations an calculations in either of both methods with still the same troubles (random tests failing, other tests green).

Only the random tests fail. The other tests all are consistently green (besides timeouts from time to time).

Also my own, local test cases all are green:

• grids with 10 rows
• grids with 9 rows
• grids with a row with 10 cells
• grids with a row with 8 cells
• grids with a number > 9
• grids with a number < 0
• unsolvable grids
• grids with several solution

@Quark_Fox I double checked, that it is the correct grid: The test case's output AND my own System.out.printlns output the same grid right before the "random" test case fails.

Thanks @Blind4Basics and @Quark_Fox for your quick answers.
I reset the trainer and tried again. Still the same issue...

[2, 8, 6, 1, 5, 9, 7, 4, 3],
[0, 5, 0, 0, 4, 8, 9, 1, 2],
[0, 1, 0, 0, 0, 0, 0, 0, 0],
[8, 0, 0, 0, 0, 5, 0, 0, 0],
[0, 0, 0, 8, 0, 4, 0, 0, 0],
[0, 0, 0, 3, 0, 0, 0, 0, 6],
[0, 0, 0, 0, 0, 0, 0, 7, 0],
[0, 3, 0, 5, 0, 0, 0, 8, 0],
[9, 7, 2, 4, 0, 0, 0, 5, 0],

My code and https://www.sudokuwiki.org/sudoku.htm says, that there is only one, valid solution:

[2, 8, 6, 1, 5, 9, 7, 4, 3]
[3, 5, 7, 6, 4, 8, 9, 1, 2]
[4, 1, 9, 7, 3, 2, 5, 6, 8]
[8, 9, 1, 2, 6, 5, 4, 3, 7]
[6, 2, 3, 8, 7, 4, 1, 9, 5]
[7, 4, 5, 3, 9, 1, 8, 2, 6]
[5, 6, 8, 9, 1, 3, 2, 7, 4]
[1, 3, 4, 5, 2, 7, 6, 8, 9]
[9, 7, 2, 4, 8, 6, 3, 5, 1]

use this string to import to sudokuwiki.org:

2,8,6,1,5,9,7,4,3,0,5,0,0,4,8,9,1,2,0,1,0,0,0,0,0,0,0,8,0,0,0,0,5,0,0,0,0,0,0,8,0,4,0,0,0,0,0,0,3,0,0,0,0,6,0,0,0,0,0,0,0,7,0,0,3,0,5,0,0,0,8,0,9,7,2,4,0,0,0,5,0

I seem to have the same issue. From the description, I checked all these criteria:

• it has 9 rows
• it has 9 cells in each row
• all values are either 0 or in the range 1-9
• there is only one solution according to my code and also according to https://www.sudokuwiki.org/sudoku.htm
• the puzzle is solvable

[3, 2, 7, 1, 5, 6, 4, 9, 8],
[9, 4, 5, 2, 3, 8, 6, 7, 1],
[1, 6, 8, 4, 9, 7, 5, 2, 3],
[8, 5, 3, 6, 7, 1, 9, 4, 2],
[2, 7, 6, 3, 4, 9, 1, 8, 5],
[4, 9, 1, 8, 2, 5, 7, 3, 6],
[7, 8, 4, 5, 6, 3, 2, 1, 9],
[5, 1, 2, 9, 8, 4, 3, 6, 7],
[6, 3, 9, 7, 1, 2, 8, 5, 4]

There seems to be something I am missing... Any hints?

You could remove the first line (empty code comment)