I have a client for which I made use of the automated thumbnail generation. When uploading a PDF, I grab the jpg file generated from the first page (introduced in WP 4.7 IIRC), and show it to the end-user on the website. This feature has stopped working.
Same thing happened on my local test server. When resetting the Image Sizes under Settings -> Media it started working again on the test page. This did not work for the client’s live page.
They use Imagify (not used on test), and when viewing the media library as list view, the PDF reads This media lacks the required metadata and cannot be optimized. I tested this with a file that had already been uploaded in the past. In the past, the PDF preview was generated without problems. When uploading the same PDF today, it reads the aforementioned message. I don’t feel comfortable saying that it is definitely related to Imagify, though.
The client runs a multi-site, with most Plugins being network activated through their parent, so testing functionality by deactivating plugins unfortunately is not an option.
Any clue how to continue the debugging of this?
Read more here:: WordPress stopped generating thumbnails from PDF