The "return" and "save and return" button on the properties page of a task might not behave properly if it is being opened as a popup.

This problem can happen if the list of tasks is sorted by any column, before selecting/clicking on any of them.
If the list of tasks have been sorted previously, the popup window showing the details of the task will not be automatically closed when clicking "return" or "save and return".

Instead of disappearing, the popup window will display the list of tasks.

Closing the window will solve the problem until opening a new task, having re-sorted the list of tasks before.

Clarity PPM 15.x

Workaround proposed:
Open the task on the same frame instead of opening it as a popup.

Click on the options wheel on the Task List portlet.
Options => Configure
List Column Sections => Fields

Observe the list of column fields.
The Task has a Link to the Task Properties.
Click on the Properties of the field.
Observe the option:
Open as Pop-up

This is marked/checked by default.
Unchek the option. 

After that, when clicking on a task, its properties will open on the same window instead of in a pop up.
Doing this, the return/save and return is always behaving properly, even after re-sorting the columns.