Assignment Instability

Hello!
Recently I have faced a disappointing problem with assigning lesson and quizzes to a course. In fact I had them assigned before and everything was going well, until 2 day ago! When I logged in to WP Dashboard and went directly to my course, I saw every lesson and quiz were missing from the course. At first I was afraid that everything might be gone, but after checking lessons and quizzes list, I saw that all the content is available but they are not assigned to any course. So I went back to the course and tried to define sections and assigning lessons to them. During the assignment everything seemed as normal as before, but after clicking “Save Course” button, the page loads and everything I assigned was gone again!
1. I have disabled any cache and compressing plugins (Autoptimize, Hummingbird), but the problem was still there. Given that previously everything was fine with these plugins, I seriously doubt 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, so the problem should not be originated from User otherness.
Thank you