Problems with project copying, creating template, or project from template

Symptoms
Errors like "Project id= with parent_id=0: parent missing or without 'lft' value" during project copying, creating template or creating project from template.

Project tree is broken. A project appears to be under a different parent project in project list and in detail view of the project.

Solution

1. Go to server where the problem is seen. Switch to user from whom redmine is running.
2. Go to your current redmine folder (default: /home/easy/current)
3. Run rails console
   bundle exec rails c RAILS_ENV=production
4. Run project_tree and issues_tree rebuild
   Issue_tree.rebuild!
   Project_tree.rebuild!
5. Check if the problem still exists! If yes, continue.
6. With sudo open file /etc/mysql/my.cnf
   sudo vim /etc/mysql/my.cnf
   and add the next line somewhere in [mysqld] area
   transaction_isolation   = SERIALIZABLE
   save the changes and restart mysql service.
7. Check. Everything should work properly now.