Samsung Galaxy S3 doesn't mount external SD card

I've recently flashed the stock firmware onto my Samsung Galaxy S3 because there was one feature which didn't work on CyanogenMod. My external SD card worked fine until recently - the 'mount SD card' option in Settings is now grayed out, and whenever I insert the SD card into the device, the following messages appear in the logcat:

W/MountService( 3370): Failed to mount media on insertion
W/MountService( 3370): java.lang.NullPointerException
W/MountService( 3370):     at com.android.server.MountService.doMountVolume(MountService.java:1212)
W/MountService( 3370):     at com.android.server.MountService.doMountVolume(MountService.java:1288)
W/MountService( 3370):     at com.android.server.MountService.access$2100(MountService.java:134)
W/MountService( 3370):     at com.android.server.MountService$6.run(MountService.java:1021)

My computer seems to recognise the SD card just fine. Can anyone help me with solving this problem?

Update 1: I was successful in mounting the SD card manually using the shell command from this tutorial: http://forum.xda-developers.com/showthread.php?t=1804573, however it was only accessible to apps with root permissions.

Update 2: As suggested in the comments section, I formatted the card in question to exFAT. The problem persists.

Update 3: The dmesg (/proc/kmsg) file seemed to have some relevant information. This is what got logged into it when I removed then inserted the card:

<6>[ 5096.384848] c2 mmc1: card removed.
<6>[ 5096.385093] c0 mmc1: card e624 removed
<6>[ 5096.478613] c0 sdhci_set_ios : MMC Card OFF samsung-hsmmc
<6>[ 5097.221780] c2 mmc1: card removed.
<6>[ 5097.221830] c2 mmc1: card inserted.
<6>[ 5097.421722] c2 sdhci_set_ios : MMC Card ON samsung-hsmmc

<6>[ 5097.450386] c0 mmc1: cmd 52 command timeout error
<6>[ 5097.450735] c0 mmc1: cmd 52 command timeout error
<6>[ 5097.454610] c0 mmc1: cmd 5 command timeout error
<6>[ 5097.454944] c0 mmc1: cmd 5 command timeout error
<6>[ 5097.455274] c0 mmc1: cmd 5 command timeout error
<6>[ 5097.455606] c0 mmc1: cmd 5 command timeout error
<6>[ 5097.599241] c2 mmc1: new high speed SDHC card at address e624
<6>[ 5097.600105] c3 mmcblk1: mmc1:e624 SU32G 29.7 GiB
<6>[ 5097.614116] c3  mmcblk1: p1

Answer

I fixed the problem just by flashing the stock firmware (using Odin) again. I didn't even have to wipe /data - all my apps are still there.