You might have a newer version of the pdftoppm utility on your new machine. It generates other file names for its output files.
Try the following patch against the DocMGR sources (it's for 0.58, but it should work equally well for older versions):
diff -urN docmgr-old/modules/center/file/object/thumbnail.php docmgr/modules/center/file/object/thumbnail.php
---...
