In my system not upgraded but installed from ISO Ubuntu 24.04 LTS "Noble Numbat" - Daily amd64 (20240215) I have the same problem but my .cache/thumbnails is as follows
corrado@corrado-n4-nn-0215:~/.cache/thumbnails$ tree -d
.
├── fail
│ └── gnome-thumbnail-factory
├── large
└── normal
gnome-thumbnail-factory contains more than 200 .png files 1 × 1 pixel
large and normal contains few .png files
In my system not upgraded but installed from ISO Ubuntu 24.04 LTS "Noble Numbat" - Daily amd64 (20240215) I have the same problem but my .cache/thumbnails is as follows corrado- n4-nn-0215: ~/.cache/ thumbnails$ tree -d -factory
corrado@
.
├── fail
│ └── gnome-thumbnail
├── large
└── normal
gnome-thumbnail -factory contains more than 200 .png files 1 × 1 pixel
large and normal contains few .png files