Having the same problem!
Images upload to one site, but not another site. Exactly the same images/WP version/Theme/plugin versions etc., on both sites.
The only difference I can see is in the Server Settings (Gallery overview). There are two differences:
1) PHP Memory Limit: One that works 256, one that doesn’t 128, and
2) PHP Max Script Execute time: One that works N/As, one that doesn’t 30s
The images upload using another Gallery Plugin, however I would rather have the same plugin on all sites.
Please advise,
Thanks, Dave