Thumbnails do not show at all

I’ve been using the plugin slideshow-gallery (Version 1.2.3.2) which uses TimThumb. Up until last week the thumbnails were showing fine. The only thing I did was trying to update the Slideshow Gallery plugin, but that caused not only the thumbs, but also the large image to disappear, so I downgraded to Version 1.2.3.2 again. When I inspect the broken images, I get the following error:
Could not create cache clean timestamp file.

I have tried changing the files’ permissions, deleting the index file in /cache and all the recommendations I found online, but the issue is still occurring. Here is an example of a gallery where the thumbs are broken: https://compagnie.voyagesgendron.com/2014/08/25/maison-bellevue-22-rue-savard-st-joachim-qc/

I also found that for a different post type (destination) the thumbnails still show fine under the Photos tab:
https://ski.voyagesgendron.com/destination/valle-nevado/

Any thoughts as to what else could be failing?

Thanks,

https://www.ads-software.com/plugins/timthumb/