In our environment every SonarQube user can browse each project. We also have some users as project admins and they could handle the other permissions for special users. With SonarQube 5.2 (and SonarQube 5.3) a project admin can't access the project permission page, he gets a "insufficient privileges". Currently only system admins can manage the project permissions.
What can we do to get the old behavior back?
You've spotted a bug. It will be fixed in 5.4: https://jira.sonarsource.com/browse/SONAR-7266