CoursePress Pro not responding after 12th Unit

We don't know why, but after we added the 13th unit the system only keeps loading and doesn't allow us to do nothing. We can't neither publich the unit or add any question fields. we already disebled all the plugins and that didn't solve the problem.

How to replicate?

1. Access wp-admin

2. Go to Coursepress > Courses

3. Add a new course

4. Go to units tab

5. Create and edit 13 units... It will keep loading forever.

We couldn't find anything related to this issue. We thought it could be because of any of the security plugins, but the trouble still appear also in and the we only have the theme and CoursePress Pro.

We also selected a wordpress default theme and didn't work.