Hi @bmakhmetov , sorry to hear about the issue.
It sounds like the issue is on submission of quizzes. Can you confirm that you’re using version 3.8.0.1, not 3.8? We released 3.8.0.1 right after 3.8 because we see an issue with LearnDash courses not completing correctly in a recipe if WooCommerce isn’t installed. That may be relevant to what you’re seeing. After confirming you’re on 3.8.0.1 and it’s happening, can you check your PHP error log to see more details about the error? If the problem is on quiz submission, it’s probably a 500 error, so the error/debug log should have more info. Can you also confirm if this quiz submission is marking a lesson or course as completed? You can also send us the error log, but it may be better to do so on our website or to [email protected] in case there’s any private info.