After noticing a huge decrease of my battery life, I used the application GSam Battery Monitor to find the origin of the problem, and discovered that the process /system/bin/sdcard was using about 90% of the phone resources. The problem persists even when removing the SD card.
How to fix this issue?
The problem was due to a folder on the internal phone memory containing several thousands of files, and causing the process /system/bin/scard to never end scanning the file system.
To find the exact directory causing the problem, I used the application 'Storage Analyser', which froze on the directory filmorago/.Log/uncaught.
Deleting the folder using the application ES Explorer took several hours due to the amount of files, and fixed the problem.
