Assignment Instability

Hello!
Recently I have faced a problem with assigning lessons and quizzes to a course. In fact I had them assigned without a problem till 2 days ago that after logging in to Dashboard, I saw all the previously assigned content (lessons+Quizzes) were missing from the course!!! At first I thought everything is gone, but after checking I found out that they are completely available from lessons and quizzes list and the only problem is the assignment. So I tried to define the new sections for the course and assign the lessons again: During assignment everything seems normal but after pressing “Save Course” button, everything I assigned is gone again!

1. I disabled any compression plugins (like Autoptimize and HummingBird), but the problem was still there. Given that there was not any problem before with these plugins, I see it highly unlikely that they are the cause of the problem.
2. I have the latest LP (v. 2.1.7.2) installed on EDUMA theme.
3. I am the only user from the beginning and everything is written by me. So the assignment problem could not originate from the user otherness.