This could be due to the Ajax (Asynchronous JavaScript and XML) query that loads the popup. Visit our guide on how to adjust this for your environment.
This is due to how reports are being created using the LearnDash activity tables in the database. The Edit Quiz Answers for LearnDash plugin is not designed to change the LearnDash activity tables as this could possibly impact things in a negative manner.
As a safer alternative, running a LearnDash data upgrade to sync the LD activity tables with the ProQuiz tables should alleviate this problem.