I’m running a database of around 1000 files, mostly PDFs. I’ve never used the feature to create thumbnails, but so far it seems to be not working for me. Ghostscript is installed on my machine and running – your pdf-thumbnail-test delivers a working thumbnail.
Nonetheless nothing happens, when I rescan my whole library (the whole thing just hangs up, my be due to too many files) or specific files. So if I rescan one PDF file it tells me, that it was successful but I can’t find any thumbnails on my server, nor do I see any on the file page for that specific file.