The Competitions Tab is working again as of Sunday, August 2!
Just submitted today's Daily Sudoku #3675 (Diabolical) successfully. Looking at my Submission History, it now shows Total Correct Submissions: 336 along with "Missed" submissions from July 19 through August 1, the period of time the Daily Sudoku Competition was unavailable.
Surprisingly, the "missed" submissions did not automatically knock me off the Roll of Honour which currently shows everyone with the usual 350 correct submissions label. Perhaps that is all part of the ongoing effort at crediting everyone for the unavailable puzzles.
Good job, Splash Software at bringing back Sudoku Online! Thank you for not disappearing for good on us.