Our Moodle (3.6) doesn't save changes to the course content

Thank you for visiting this project. We have a problem with our Moodle install and would like your help to fix it. Our troubleshooting efforts lead us to believe that we may be having session handling issues. Currently, our Moodle set up will not save changes to any setting in the server administration area, or to course content. Somehow grading works okay at the moment and people are able to upload files. Also, the Moodle developer who helped us in a previous project believes it is an issue with NGINX or the server, not with the Moodle install because the copy he made of our Moodle install on his server works flawlessly. Our moodle install was recently upgraded from 2.9 to 3.6 with the help of a fellow freelancer. It's running on PHP 7 on Linux/Apache. It did not have these issues in 2.9. In Server Administration > Server > Environment, all the server checks are green except for 2 warnings: slasharguments and mysql_full_unicode_support The project will be considered complete when: - we can edit and save course content and - we can edit and save changes to site administration settings - I can log out and back in without any errors We would also like to discuss a solution to notify our email automation platform of milestones that a student reaches through the course, for all 3 of our current courses. We will only consider bids from bidders who can give us some ideas about that. Thank you.