Generate thumbnails without user specified fails afer 1 user

This works: 

php make_thumbs.php user2

But I want to create thumbs for all users. This used to work, now it works for user1, fails for user2 and stops:

php make_thumbs.php

The output:

-------------------- START ----------------------
Image size: 400 x 400
Generating thumbnails for user1's files:

Getting directory listing...: 42,181 files found

Processing files:
[...]

32659 files processed.

Generating thumbnails for user2's files:
User not found!