Clicking on the Set Flag or Clear Flag button results in the following error:

Starting in version 4.42 of Case Flags, Vicasso has implemented a change within the product to more strictly enforce sharing.

This error indicates that the user does not have Edit access to the "Case Flag Date/Time" field on the Case object. To provide the user with sufficient access, you can choose either to assign a permission set to a user or to a profile.

Method 1: Assign Permission Set to User
Case Flags comes packaged with a permission set called "Case Flags End User," and this includes sufficient access to the Case Flag Date/Time field. This permission set can be found and assigned to end users under Setup | Administer | Manage Users | Permission Sets. 

Method 2: Assign Permission Set to Profile
Access to the Case Flag Date/Time field can be provided directly at the Profile level. To do this, navigate to Setup | Administer | Manage Users | Profiles, and click the Profile to which you'd like to extend access. Next, follow the directions below based on your user interface.

Standard User Interface:

1. Scroll down the page to the section titled "Field-Level Security", and click the View link next to the Case object.
2. Click Edit at the top of the Page.
3. Scroll down to the field titled "Case Flag Date/Time" and ensure both the Read Access and Edit Access boxes are checked.
4. Click Save at the top of the page.
5. Repeat this process for any additional profiles that will be using the Set Flag and Clear Flag buttons.

Enhanced User Interface:

1. Click the link for Object Settings, and select the Case object on the following page.
2. Click Edit at the top of the Page.
3. Scroll down to the field titled "Case Flag Date/Time" and ensure both the Read Access and Edit Access boxes are checked.
4. Click Save at the top of the page.
5. Repeat this process for any additional profiles that will be using the Set Flag and Clear Flag buttons.