no thumbnails for pdfs

Hi! I’ve suddenly discovered that pages with pdfs galleries don’t have thumbnails anymore. You can see it here, for example: http://thinkcognitive.org/ru/blog/rezul-taty-konkursa-luchshij-poster-shkoly-po-versii-tcts-2015 (it’s in Russian, pdf gallery is at the bottom). I tried removing & reinstalling Document Gallery = it doesn’t help. There are no errors on logging page. There are messages like that though: (DG_Thumb::initThumbs) Populating thumbnail cache. Here’s the info from Advanced tab:
Array
(
[thumber] => Array
(
[gs] => /usr/bin/gs
[active] => Array
(
[av] => 1
[gs] => 1
[imagick] => 1
[thumber-co] =>
)

[width] => 200
[height] => 200
)

[thumber-co] => Array
(
[uid] =>
[secret] =>
[subscription] => Array
(
)

[direct_upload] =>
[mime_types] => Array
(
)

)

[gallery] => Array
(
[columns] => 4
[fancy] => 1
[order] => ASC
[orderby] => menu_order
[relation] => AND
[limit] => -1
[mime_types] => application,video,text,audio,image
[paginate] => 1
[post_status] => any
[post_type] => attachment
[attachment_pg] =>
[descriptions] =>
[new_window] =>
[skip] => 0
)

[css] => Array
(
[text] =>
)

[meta] => Array
(
[version] => 4.2.3
[items_per_page] => 10
[donate_link] => https://www.paypal.com/cgi-bin/webscr?cmd=_donations&business=EE5LWRLG933EN&lc=US&item_name=Document%20Gallery%20Plugin&item_number=document%2dgallery¤cy_code=USD&bn=PP%2dDonationsBF%3abtn_donateCC_LG%2egif%3aNonHosted
)

[logging] => Array
(
[enabled] => 1
[purge_interval] => 7
)

)

https://wordpress.org/plugins/document-gallery/