I'm posting this here because I ran into that error earlier. I'm using some code to copy images from one upload folder to the wordpress uploads folder (works fine locally, pretty sure it worked like 2 weeks ago here too). Tonight when I started testing, it started copying images but wouldn't resize my thumbnails, then I got the 503 error, so I tried the thumbnail regeneration fix. I haven't run into the 503 since, but now the copied files are all 0 bytes. I tried checking my folder permissions, even setting everything to 777, and it's still not working... Any ideas?