After a serious research (time waste
) I can say that it was definitely no space problem.
Seems to me that filefox uploads a file to a temp folder and after successful upload and crc check it moves it to my affiliate drive, only to realize that there is not enough space to move the file.
Instead of responding with not enough space message, I presume API simply returns bogus link.
After a big file fails, the small file goes through (since there is just enough space for it), and than next big file fails.
It's FileFox Api that has a problem and I guess I'll mail them as soon as I figure out exactly what to say...
Meanwhile I'll keep in mind to clean my drive of old files more regularly.