Per-user directories fail when UID contains '0'

I have created a folder called disknode inside my files folder. It is owned by and in the group of the same user as the Apache user and is set to 777 for now. Our author and editor groups have been given permission to use disknode in all ways except escaping the user directory.

As most users, I try uploading, and it works.

I have a single user, the one who uploads the most info on the site, for whom it will not work. Instead it says:

* The selected file /tmp/tmp_V0XcjT could not be uploaded, because the destination is not properly configured.
* Failed to save disknode/1/0/10/file.pdf
* disknode/1/0/10 is not writeble. The directory will be created when needed.

I tried some other things, and found that users 10, 20, and 30 have this problem, but users 11, 21, 31 and others not ending in 0 do not.

Any help you can provide tracking this down would be greatly appreciated.