Thank you, Tim. I did, but the problem persists. I am now trying to revert to a much older version of Moodle.
We faced same problem while adding random question from question bank.
1) We were facing problem since we upgraded to 3.5
2) Searched entire net but could not find any solution
3) Then upgraded to 3.6.3 BUT PROBLEM PERSISTED
4) On SERVER ENVIRONMENT no error was showing , everything showing perfect
5) By try and error method, one by one, we chanced upon a solution of changing MYSQL or MariaDB we changed (in config file) utf8mb4_general_ci to utf8_general_ci
6) PROBLEM SOLVED
7) But under SERVER ENVIRONMENT it shows a POTENTIAL PROBLEM as per screenshot. As we were doing try and error method one by one.... we ignored it and checked adding random question from question bank.... AND IT WAS WORKING.....
8) We tried above method on all our installation and it is working fine... we can add random question from question bank without any error.
9) We have NOT fully tested it other than adding random question from question bank.. If we will find any issue anywhere else due to above rectification, we will post it here..
10) We encourage all to post their findings and results by applying above method.
Thank you, Arkin Institute!
However, I cannot try this, because I needed to solve the problem quickly. So I deleted the old installation and clean installed moodle. It now works perfectly.