I couldn't update OTA either - I was blaming it on rootfs, so started uninstalling. Tried when I got up to 48.6, and updated fine (though so far wishing I hadn't, actually).

Based on that, I wonder if there are certain installations which are incompatible: try uninstalling any themes - I removed all mine. Other than that, I removed most Ovi stuff (as it has a reputation for poor rootfs use, though I didn't see much difference) and a few games.

I'd be interested to know what, if anything, others had to do to update OTA.