In T337649#9378880, @hnowlan wrote:

We recently re-raised concurrency on the thumbnailrender job as it appeared backlogs had dropped causing initial issues. This had a knock-on effect related to T338297 which caused erroneous rate limits to be applied. This has hopefully reduced somewhat, please let us know.

Another update by a layman user: not only the page previews for PDF indices on Wikisource have been completely down for the past few days, but the bug also starts affecting page previews on Commons. Some of my latest uploads are completely unable to be previewed, yet everything is completely normal when I click into the involving PDF file. Not only that, OCR is also available to the affected pages.

As a normal user who know nothing about the subliminal coding, I'd like to report that the problem seems to be alleviated for the past few days. At least I don't need to push the button for some 10 or 15 times, and in general it only takes 2-3 times for Google OCR to identify the text. Did you changed something for the OCR gadget?

Not sure how these information are useful to you, but Soda suggest that my troubles in T338100 may relate to the same problem. From my experience all 3 OCR gadgets have the same problem as well.