Can’t upload larger images since 3.4

Shortly after installing 3.4, larger image files nearly always fail to upload with alternate sizes, and the full-size option registers as “0x0”. This results in thumbnail galleries of overlapping, full-sized photos.

If I re-upload a photo enough times, sometimes it’ll finally take. But mostly not.

Sometimes the photos have metadata, sometimes not. It doesn’t seem to matter.

Sometimes the upload process stops and I’m asked to log in again before it finishes.

This failure to upload properly happens both with the multi-file uploader and the single-file browser uploader.

I’ve tried deactivating my plugins, switching themes to Twenty Eleven, increasing memory allotments in the php.ini file, clearing the cache, using different browsers, and different computers (both of them Macs). I’ve had my web host check out the wp_postmeta database to make sure it wasn’t locked.

Has anyone else had this problem since 3.4 and found a fix?