SUMMARY:
Whey you try to update tasks on a project using the CSV import option in Web TimeSheet.
On the CSV Import page the name of tasks in the "Task Name Level 1" column may be displayed in red color.
When you hover the mouse pointer over the red icon in the beginning of the row the following error may be displayed.
 
Editing project 'X', 'Y' is invalid input for task.  
 
Where X is the project name and Y is the task name.
 
SOLUTION:
The issue is resolved by entering the full path of the task in the "Task Name Level 1" column.  That is if you are trying to add task 'Y' to project 'X'  then you need to mention the task name as "Project X / Task Y"(without double-quotes) in the "Task Name Level1" column.