Sequentially check the rights setup.
1. Check if UserA has the following global rights:
a. Global right(Timesheets - Navigate)
b. Global right(Timesheet - Approve - All)
2. Check if UserA has instance right(Resource - Enter Time) for UserB
3. Navigate Administration > Resources > userB > Access to this Resource > Full View
Check if there is the right(Resource - Enter Time) which is granted through an OBS
If there is and the request is to remove the right, go to Administration > OBS > selected the OBS > Units > drill down and select the unit
Click on the "Access Right for Unit" tab
and removed the right.