Session files with 0 bytes

Hello everyone! I'm experiencing some problems with session files. Recently, CodeIgniter started creating session files with 0 bytes, two or three every second. Sometimes 5 or 6, even more.

The app is working perfectly; the only problem is that the large amount of files causes the whole system to break when the session folder reaches 15.000 files... only a few of them containing actual session data.

Could you give some insight regarding why this is happening, and any clue to solve it?

Thank you in advance.