Internal memory (sdcard0) not writable - Android 4.3 - Xperia T (LT30p)
I finally fixed it. In case someone has the same problem in the future, here's what I did:


After googling here and there about how to completely wipe an Android phone, I read that to do that you must encrypt your data first and then do a factory reset.


So, I still don't know what caused it, I only know that now it's fixed.