Confirming.

The error message is

2014-09-11 21:57:33 mw1156 commonswiki: thumbnail failed on mw1156: error 1 "**** Warning: considering '0000000000 XXXXX n' as a free entry.

• Warning: considering '0000000000 XXXXX n' as a free entry.

(...line repeats 122 times...)

• Warning: considering '0000000000 XXXXX n' as a free entry.

convert: no decode delegate for this image format `/tmp/magick-pF48VPYV' @ error/constitute.c/ReadImage/532.
convert: missing an image filename `/tmp/transform_4ffaf3e633a6-1.jpg' @ error/convert.c/ConvertImageCommand/3011." from "('gs' '-sDEVICE=jpeg' '-sOutputFile=-' '-dFirstPage=1' '-dLastPage=1' '-r150' '-dBATCH' '-dNOPAUSE' '-q' '/tmp/localcopy_3d29485c72ff-1.pdf' | 'convert' '-depth' '8' '-quality' '95' '-resize' '422' '-' '/tmp/transform_4ffaf3e633a6-1.jpg')"

Both evince and xpdf take a long time to display the first page of the pdf file (Although they eventually do display it). Maybe something funky with this particular file?

I tried to upload it again to test but nothing changes. :/

As an update, I tried to overwrite the image by getting a new cover to the pdf. While it now opens correctly in preview on my Mac, it still doesn't generate the thumbnails (I also uploaded it at https://commons.wikimedia.org/w/index.php?title=File:Welcome_2WP_Serbian_04-16-2012_.pdf&page=1 , and this one actually gave me some "too many pixels" error message).
Can you recommend the best way to proceed? Thanks.

In T72734#764455, @Tgr wrote:

convert: no decode delegate for this image format

More testcases: Same problem for
https://commons.wikimedia.org/wiki/File:WorldAviation.198409.BackCover.pdf
https://commons.wikimedia.org/wiki/File:WorldAviation.MiddleTwoPages.ICAORegions.pdf
brought up in https://commons.wikimedia.org/w/index.php?title=Commons:Village_pump&oldid=144583567#No_thumbnails_from_pdf_files.3F_Like_File:WorldAviation.198409.BackCover.pdf

In T72734#946929, @Aklapper wrote:

In T72734#764455, @Tgr wrote:

convert: no decode delegate for this image format

More testcases: Same problem for
https://commons.wikimedia.org/wiki/File:WorldAviation.198409.BackCover.pdf
https://commons.wikimedia.org/wiki/File:WorldAviation.MiddleTwoPages.ICAORegions.pdf
brought up in https://commons.wikimedia.org/w/index.php?title=Commons:Village_pump&oldid=144583567#No_thumbnails_from_pdf_files.3F_Like_File:WorldAviation.198409.BackCover.pdf

I looked into that file. File took insanely long (about 10 minutes) for ghostscript to render. My guess is ghostscript took more than the $wgMaxShellTime limit, thus gs died and sent convert something that was not a file. image magick gave that error. I don't know if there's anything we can realistically do in such a situation. We probably do not want limits being what would be required in order to render this file (Assuming I'm right in that is what is causing this).

In T72734#952161, @Bawolff wrote:

In T72734#946929, @Aklapper wrote:

In T72734#764455, @Tgr wrote:

convert: no decode delegate for this image format

More testcases: Same problem for
https://commons.wikimedia.org/wiki/File:WorldAviation.198409.BackCover.pdf
https://commons.wikimedia.org/wiki/File:WorldAviation.MiddleTwoPages.ICAORegions.pdf
brought up in https://commons.wikimedia.org/w/index.php?title=Commons:Village_pump&oldid=144583567#No_thumbnails_from_pdf_files.3F_Like_File:WorldAviation.198409.BackCover.pdf

I looked into that file. File took insanely long (about 10 minutes) for ghostscript to render. My guess is ghostscript took more than the $wgMaxShellTime limit, thus gs died and sent convert something that was not a file. image magick gave that error. I don't know if there's anything we can realistically do in such a situation. We probably do not want limits being what would be required in order to render this file (Assuming I'm right in that is what is causing this).

I can reupload them after optimization. I do not know how to do that though...