The WordPress plugin iThemes Security (previously known as Better WP Security) has a known conflict with CoSchedule. However, it can be easily fixed with a change to the plugin's settings.
If iThemes Security is installed, the "Filter Suspicious Query Strings" tweak will cause problems in CoSchedule. This can be resolved by doing the following:
1. Click on Better iThemes Security's "Settings" tab.
2. Choose "System Tweaks" from the drop-down list.
3. Make sure the "Filter Suspicious Query Strings" option is not checked.
Click Save All Changes.