Just to clarify and elaborate a bit more of what I said that I'm afraid some parts were not clear.
To clear things up:
We have a group of about 20 users, some of whom use Excel 2010 and some others use Excel 2013.
There's only 1 user that experiences this exact problem and she's on Excel 2010.
To elaborate more:
Most users including the one experiencing this problem use EPM Add-in version 10.0 SP 21 Patch 4. And I just made sure that updating EPM Add-in to the latest version -- version 10.0 SP 23 Patch 1 -- didn't help.
So it seems to me that it's a user-specific problem. and NOT a system-specific, a software version-specific, nor a machine-specific problem. And I'm now focusing on all the user-specific settings to find out what causes it.